Technical Abstract: The objectives of this study were to evaluate hCG in place of GnRH and the effects of 48-h calf removal (CR) on pregnancy rates of cows synchronized with the CO-Synch protocol. Suckled beef cows (n = 467) within two locations were assigned to receive either GnRH or hCG with or without CR in the CO-Synch protocol. On d 0 and d 9, cows received either 2500 IU of hCG or 100 mu g of GnRH i.m. and on d 7 all cows received PGF2alpha. At one location, blood samples were collected from all cows on d -14, -7, 0, 7, 9, & 16. Serum from each sample was analyzed for progesterone level to identify cyclic and anestrous cows (d -14, -7, & 0) and corpora lutea presence (d 7), regression (d 9), and ovulatory response(d 16). Calves were removed from half of the cows that received GnRH or hCG on d 7 and returned on d 9. Visual observation (3x daily) was used to identify estrual cows. Cows that exhibited estrus were bred 12 h later, and those not observed in estrus by d 9 were mass mated and received a second injection of either GnRH or hCG. Pregnancy status was determined at both locations 40 d following timed AI. The AI pregnancy rate for GnRH treated cows with and without CR and hCG treated cows with and without CR was 46%, 50%, 35%, 34%, respectively. The hCG treated cows (34%) had a lower (P<.05) pregnancy rate than GnRH treated cows (48%) regardless of CR. More (P<.05) hCG treated cows exhibited short estrous cycles following timed AI. In addition, there tended to be more (P=.2) hCG treated cows with elevated progesterone at the timed AI. Temporary CR had no effect (P>.1) on AI pregnancy rate of cows within hormone treatments. We conclude hCG is not a suitable replacement for GnRH to synchronize ovulation with the CO-Synch protocol.