The objectives of this experiment were to evaluate the effect of various estrus synchronization programs on estrus detection rate (EDR) and pregnancy rate (PR) in lactating cows. Spring-calving, lactating dairy cows (n = 3,096) were allocated to one of six treatments: 1) PG (n = 514), estrus detection for 10 d before the onset of the breeding season (BS), cows detected in estrus received PGF2 alpha (PGF) 8 d after estrus, and all cows to be bred by d 11 of BS; 2) PG+EBZ (n = 510), as in PG plus cows treated with PGF received 1 mg of estradiol benzoate 48 h later; 3) C (n = 522), progestogen (controlled intravaginal drug release; CIDR) inserted per vaginum 10 d before BS for 8 d, PGF treatment on the day before CIDR removal, and AI within 6 d after CIDR withdrawal; 4) C+EBZ (n = 520), as in C, plus 10 mg of estradiol benzoate 10 d before BS; 5) C+BUS (n = 517), as in C plus 10 micrograms of buserelin on the day of CIDR insertion; or 6) CON (n = 513), no treatment, and a 32-d period of AI. Relative to other CIDR-treated groups, estradiol benzoate at the time of CIDR insertion reduced (P < .05) the EDR (C, 85.0%;C+EBZ, 75.9%; C+BUS, 88.5%). The PR to first AI was reduced (P < .05) by C compared with other treatments (PG, 60.9%; PG+EBZ, 57.2%; C, 46.6%; C+EBZ, 60.5%; C+BUS, 57.9%; CON, 60.1%). The interval from the onset of BS to AI and pregnancy was reduced (P < .05) by up to 9 d by estrus synchronization relative to controls (PG, 5.5 +/- .2 d; PG+EBZ, 5.2 +/- .2 d; C 1.7 +/- .1 d; C+EBZ, 2.3 +/- .1 d; C+BUS, 106 +/- .04d, CON, 10.4 +/- .3 d). In conclusion an 8-d progestogen-buserelin-PGF treatment resulted in the best overall estrus detection and pregnancy rates, which would be beneficial to a compact calving program.