Introduction: Isoflavones also known as phytoestrogens are naturally synthesized phenolic components of plants that have either estrogenic or antiestrogenic effects (Křížová et al., 2019). A variety of phytoestrogens have been shown to bind to both isoforms of the estrogen receptor (ERα and ERβ) in vitro and activate ER-dependent gene transcription (Patisaul et al., 2001). There are three main classes of phytoestrogens: (1) isoflavones [derived principally from soybean, i.e. soybean isoflavone (SIF)]; (2) lignans (found in large quantities in flaxseed), and (3) coumestans (derived from sprouting plants like alfalfa). SIFs have become a hot issue in the last 20 years due to the excessive consumption of soy-derived products in animal feed (Křížová et al., 2009). Genistein, one of the most active flavonoids of soybean, exerts numerous biological effects including antioxidation, chemoprevention, antiproliferation, and anticancer (Messina, 1999). Recently, the interest in genistein action has grown because of its role as a factor that can modulate processes involved in carbohydrate and lipid metabolism by alterations in the hormonal status of the organism (Szkudelska and Nogowski, 2007). Moreover, isoflavones have gained popularity due to their positive effects on human health as an alternative therapy for menopausal symptoms, osteoporosis, and some hormonal disorders (Křížová et al., 2009). On the other hand, these compounds may be considered endocrine disruptors due to the negative effects found in the reproduction of animal models (Opałka et al., 2004). It was reported that soy isoflavones antagonize estrogen-induced behavior in the rat brain (Patisaul et al., 2005). In birds, isoflavones and other phytochemicals may, in part, regulate seasonal reproductive cycles through the inhibition of reproductive behavior (Panzica et al., 2005). The dietary supplementation of soy isoflavones to growing Japanese quail decreased testis size (Wilhelms et al., 2006) but did not affect testis growth stimulated by photoperiod in male songbirds (Junco hyemalis) (Corbitt et al., 2007). The dietary level of soy isoflavones is very decisive in how these compounds affect the development of the reproductive system in birds (Boonnoon et al., 2016). On the other hand, Isoflavones and other phytochemicals may partially regulate the seasonal reproductive cycles of birds by inhibiting reproductive behavior (de Man and Peeke, 1982; reviewed in Rochester and Millam, 2009). Therefore, in the present study, the effects of dietary supplementation of soy isoflavones on the performance, sexual behavior, and plasma levels of some hormones were investigated in male Japanese quail. Materials and methods: A total of 80 male Japanese quails were purchased at 21 d and randomly divided into four treatment groups with four repetitions of 5 quails each. The birds after one week of gradual adaptation to the new rearing system and feed, received the same basal diet with soy isoflavones supplemented at 0 (control), 0.36, 0.73 and, 1.10 g/kg feed. Soyagol tablets (Goldaru pharmaceutical company) were used as a source of soy isoflavones. In this way, the tablets after powdering were added to the basal diet at the levels of 3 6, and 9 g per kg diet. At 47 days of age, six birds per treatment were selected for blood collection, and plasma levels of testosterone and T3 hormone were determined by commercial kits. At the end of the feeding trial (49 days of age), 6 birds from each treatment group were slaughtered to make measurements of carcass cut yield and some internal organs. Feed consumption, weight gain, and feed conversion efficiency were calculated weekly during three experimental periods. During the last week of the trial, appetitive male sexual behavior was assessed by measuring a learned social proximity response, and consummatory behavior was assessed by measuring copulatory behavior per se (Castagna et al 1997). The size of the cloacal glands was also recorded at the beginning and end of the experiment using a digital Vernier caliper. This research was designed using a completely randomized design. The data were analyzed using the GLM procedure of SAS software (SAS Institute, 1999). Where treatment effects were found (P< 0.05), means were separated by Duncan's multiple range tests. Results and discussion: The results showed that soybean isoflavones supplementation at the level of 1.08 g/kg diet, in the entire experimental period, led to a significant decrease in the rate of body weight gain and feed intake of birds compared to the control group (P<0.01). Dietary Supplementation of soy isoflavones at 0.72 g/kg diet, also caused a significant decrease in the feed intake of quails compared to the control (P<0.01). The experimental treatments had no significant effect on the relative weight of heart, liver, and carcass yield. There was no mortality during the trial period. The inclusion of soy isoflavones at 1.08 g/kg diet, significantly diminished plasma testosterone and increased plasma T3 levels compared to the control (P<0.01). In the same dosage, the frequency of appetitive and consummatory aspects of sexual behaviors was also reduced compared to the control group (P<0.01). Increasing the dose of isoflavone compounds in the diet of male quails also exhibited decreasing the somatic testicular index and the size of the cloacal gland in a dose-dependent manner (P<0.01). Conclusions: It is concluded that soy isoflavones may induce an inhibitory effect on male sexual behavior in Japanese quail by acting as an endocrine disruptor and inhibiting reproductive development. Soy isoflavones in doses of 0.36 and 0.72 g/kg diet, even though they were much higher than the usual doses used in previous studies, did not significantly affect the body weight gain and relative weights of the liver in Japanese quail. Therefore, it can be concluded that isoflavones did not show severe toxic and hormone-disrupter effects at these supplementation levels. However, isoflavones in the high dose used in the present study decreased significantly plasma testosterone concentrations and led to dramatic changes in the testes weights and sexual behavior of male Japanese quails. The present results suggest that high concentrations of isoflavones in the diet may exert endocrine disruptor-like effects on the reproductive system of the male, but not in the female quail. 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