Technical Abstract: This study was conducted to evaluate ovarian and estrous responses among cows synchronized with a GnRH-PGF protocol as affected by day of the estrous cycle at the initiation of treatment. Multiparous, suckled beef cows (n=56) were fitted with HeatWatch electronic estrous detection monitors 22 d before initiation of synchronization treatment. Synchronization treatment included an injection of GnRH (100 ug) on d - 7, an injection of PGF (25 mg) on d 0 followed by 5 d of estrous detection, and AI approximately 12 h after initial observation of estrus. Ovaries of cows were scanned daily via transrectal ultrasound from d -12 until time of insemination to evaluate ovarian response to the GnRH and PGF injections. Ovarian and estrous response to the GnRH and PGF injections were dependent on day of the estrous cycle when the protocol was initiated. Among all cows, 67% responded to the GnRH injection by initiating a new follicular wave and developing a CL. Moreover, 88% responded to the PGF injection by exhibition of estrus by d 5 post- PGF. Ovarian response in cows that were between d 15 and 17 of their estrous cycle at the initiation of synchronization (d -7) was poor to both GnRH and PGF administration (14 & 14%, respect.). These cows also exhibited estrus 11+_19 h before injection of PGF. Cows classified as anestrous at the beginning of the study exhibited estrus earlier (P<.05) than cyclic cows (40.2+_16 h & 56.3+_39 h, respect.). Synchronized pregnancy rate was 71% for the 5 d breeding period. We conclude that the Select Synch protocol is capable of synchronizing estrus among the majority of cows in a herd and emphasize that accurate estrous detection beginning 24 to 30 h before the PGF injection is important.