The development of estrus synchronization protocols that facilitate fixed time artificial insemination (FTAI) in beef cattle has resulted in variable pregnancy rates. This variability has been attributed to different factors such as expression of sexual behavior, and follicular dynamics. The objective of the present study was to assess the onset of estrus response, estrus length, follicular growth, and size at AI using the 5 d-CO-Synch + CIDR program. One hundred and 3 females (81 cows and 22 heifers) were synchronized with an administration of 100 µg of GnRH IM and an intravaginal controlled internal drug release (CIDR) at d −5. At d 0, the CIDR was withdrawn and 2 doses of PGF2-α (25 mg) were administered via IM 6 h apart. Animals were observed continuously for behavioral signs of heat after CIDR removal until either 56 h (FTAI for heifers) or 72 h (FTAI for cows). At AI all animals received 100 µg of GnRH IM. Pregnancy diagnosis was performed approximately 60 d after FTAI. Results were analyzed by PROC MIXED procedure using a 2 × 2 factorial arrangement (pregnancy status and expression of heat). No differences where observed between heifers and cows. Overall, the proportion of animals showing sexual behavior was 45% (46/103), where 22% (10/46) did not get pregnant. No differences (P > 0.05) were detected between animals that showed estrus and became pregnant (n = 36) or showed estrus and were non pregnant (n = 10) at onset of sexual receptivity (52:09 h vs 46:30 h) and length of estrus (6:05 h vs 6:36 h) respectively. Non pregnant animals without estrus expression had the smallest follicle size at AI (P = 0.04), however no differences were observed in follicular growth. Pregnancy rate at FTAI was similar among animals regardless expression of estrus. In conclusion, the lack of reproductive response in animals submitted to a FTAI synchronization program can be related to a reduction in follicular size at FTAI and absence of estrus behavior.