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The objective was to evaluate the reproductive performance after oestrus synchronization using controlled internal drug release (CIDR) maintained into the vagina either 7 or 9 days. A controlled randomized block design was performed. Cows (n=255) were blocked by lactation number and days in milk and then randomly divided into 2 groups. In the first group, CIDR was maintained for 7 days (CIDR-7; n=127), and in the second group, CIDR was maintained for 9 days (CIDR-9; n=128). Both groups received Prostaglandin-F2α at CIDR removal. Oestrus response, oestrus onset, oestrus duration, conception rate, and pregnancy at days 28 and 45 were evaluated. Cows were oestrus detected 4 times a day for 120 hours after CIDR removal, and only cows in standing oestrus were artificially inseminated between 6 and 12 hours after oestrus detection. Oestrus response for both groups was 79.5% and 87.5% for CIDR-7 and CIDR-9, respectively (P=0.09). The oestrus onset (mean ± SD) for the same groups was 56.3±17.2 h and 59.4±17.9 h, respectively (P=0.19). Oestrus duration was 11.3±7.3 h and 11.7±7.8 h for groups CIDR-7 and CIDR-9, respectively (P=0.71). At day 28, the conception rates for the groups CIDR-7 and CIDR-9 days were 51.5% and 45.5% (P=0.39), and the pregnancy rates were 40.9% and 39.8% (P=0.89) for the same groups, respectively. No differences in oestrus performance and fertility in lactating cows’ oestrus synchronized by using CIDR maintained in the vagina either 7 or 9 days were detected.