To determine the efficacy of reducing the dosage of GnRH used in a protocol for synchronization of ovulation and timed AI, primiparous and multiparous lactating Holstein cows (n = 237) were randomly assigned to 1 of 2 treatment groups. Ovulation was synchronized for cows in the first group using intramuscular injections of GnRH and PGF2 alpha as follows: Day 0, 100 micrograms GnRH; Day 7, 25 mg PGF2 alpha; Day 9, 100 micrograms GnRH. Ovulation was synchronized in the second group of cows using the same injection schedule and dosage of PGF2 alpha but only 50 micrograms GnRH per injection. All cows underwent a timed AI at 12 to 18 h after the second GnRH injection. The proportion of cows ovulating in response to the second GnRH injection (synchronization rate) and pregnancy status at 28 and 56 d post AI were determined using transrectal ultrasonography. The synchronization rate, double-ovulation rate, conception rate at 28 and 56 d post AI, and pregnancy loss from 28 to 56 d post AI did not differ statistically between treatment groups. For all cows, synchronization rate was 84.0%, and double-ovulation rate was 14.1%. Conception rates calculated using all cows receiving synchronization of ovulation were 41.1% at 28 d and 34.4% at 56 d post AI. Conception rates calculated for only synchronized cows were 47.6% at 28 d and 40.1% at 56 d post AI. For all cows, pregnancy loss from 28 to 56 d post AI was 13.5%, with an attrition rate of 0.5% per day. Estimated savings in hormone costs using 50 rather than 100 micrograms GnRH per injection for synchronizing ovulation were $6.40 per cow and $20.27 per pregnancy. Thus, decreasing the dosage of GnRH used for synchronization of ovulation and timed AI in lactating dairy cows reduces synchronization costs per cow and per pregnancy without compromising the efficacy of the synchronization protocol.