Lactating dairy cows have poor reproductive efficiency because of low fertility and low rates of estrus detection. To eliminate the dependence on detection of estrus, we have recently developed a timed artificial insemination (AI) protocol that synchronized the time of ovulation using GnRH and PGF2 alpha. The effectiveness of this protocol as a management tool was compared with standard reproductive management. Lactating dairy cows (n = 333) from three herds were randomly assigned at parturition to two groups. Control cows were managed according to the typical reproductive strategy of the farm that relied on detection of estrus, the a.m.-p.m. breeding rule, and periodic use of PGF2 alpha. Treated cows had timed AI after synchronization of ovulation with GnRH and PGF2 alpha. For both groups, the voluntary waiting period was 50 d postpartum. Pregnancy diagnosis was performed by ultrasound between 32 and 38 d post-AI. Nonpregnant cows were inseminated again using the original treatment until diagnosed as pregnant or until culled from the herd. Median days to first AI (54 vs. 83) and days open (99 vs. 118) were lower for treated cows than for control cows, respectively. Pregnancy rates for the first AI were similar (37% vs. 39%) for the two groups even though treated cows were bred at an earlier time postpartum. More treated cows than control cows were pregnant at 60 d (37% vs. 5%) and at 100 d (53% vs. 35%) after calving. Thus, this protocol allowed effective management of AI in lactating dairy cows without the need for estrus detection.