Technical Abstract: Primiparous (n = 76) and multiparous (n = 328) lactating Angus cows were randomly assigned to receive either a 50 mcg or 100 mcg dose of GnRH at either the first or second GnRH injection of the CO-Synch estrus synchronization protocol to evaluate the efficacy of a reduced dosage in inducing a fertile ovulation. The result was four separate treatment groups: 50:50, 50:100, 100:50, and 100:100. The CO-Synch protocol traditionally consists of GnRH on d 0, followed by an injection of PGF on d 7. On d 9 cows were injected with another GnRH injection and mass inseminated. On d -10, and again on d 0, blood samples were obtained to examine progesterone levels and determine the proportion of anestrous cows. Calves were removed for a 48 h period on d 7 and were put back with the cows immediately after breeding. Clean- up bulls were turned in with the cows on 14 d following AI. Cows were pregnancy checked via transrectal ultrasonography 68 d after AI. Serum progesterone levels indicated that 34.9% of the cows were estrual at the start of breeding. All treatments were equally effective (P > 0.05) in inducing anestrus cows to ovulate and become pregnant. There was no difference in pregnancy rates among the treatment groups (50:50 = 50.7%; 50:100 = 49.3%; 100:50 = 50.3%; and 100:100 = 44.6%, P > 0.05), thus indicating that a reduced dosage of GnRH is effective in inducing fertile ovulation in the CO-Synch timed insemination protocol. We conclude that using a lower dose of GnRH is an effective method of reducing cost of synchronization with the CO-Synch protocol.