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Inhibins are important ovarian hormones that control the action of the pituitary gonadotropins which regulate the ovarian cycle. The present experiment aims to study the role of steroid-free follicular fluid-antibodies (S-FBFF-ab) on the reproductive hormone levels at estrus and metestrus phases in cycling female rats. Follicular fluid was collected from mature ovarian follicles, centrifuged and treatment with activated charcoal. S-FBFF was isolated and used for immunization of adult male rabbits. After 5 injections, blood was withdrawn for separation of S-FBFF-ab. Eighty mature female rats were assigned into control and treatment groups (40 each). At late metestrus phase, females of the control were injected intra-peritoneally with 100 μL of physiological saline, whereas treated females were injected intra-peritoneally with 100 μL of S-FBFF-ab. At early estrus and early metestrus phases of each estrous cycle, serum concentrations of inhibin-B, follicle stimulating hormone (FSH), estradiol (E2), and prolactin (PRL) were estimated. The results revealed that S-FBFF-ab has inhibin immuno-neutralization effect, which caused elevation in the concentrations of serum FSH and E2, and significant decline of serum PRL and inhibin-B, at early estrus, whereas significant elevation of E2 and PRL, and significant decline of serum FSH, at early metestrus. Furthermore, the results of gene expression revealed a significant elevation of ovarian aromatase gene at both early estrus and early metestrus phases, whereas pituitary PRL gene showed significant decline at early estrus phase and significant elevation at early metestrus phase. In conclusion, passive immunization against S-FBFF could augment the reproductive efficiency through increase reproductive endocrine activity.