Human chorionic gonadotrophin (hCG) plus PGF2 alpha was compared with GnRH plus PGF2 alpha for estrus synchronization of dairy cows. There were 3 treatments: GnRH analog (Buserelin, 12.6 micrograms) plus PGF2 alpha analog (Cloprostenol, 150 micrograms) 6 d later (GnRH + PGF[Day 6]); hCG (2000 IU) plus PGF2 alpha 9 d later (hCG + PGF[Day 9]); and hCG plus PGF2 alpha 6 d later (hCG + PGF[Day 6]). Treatment occurred either Days 55 to 90 or Days 91 to 135 post partum. For responses during the first 10 d after PGF2 alpha administration, estrus synchronization (P = 0.24), efficacy (percentage of treated pregnant; P = 0.20) and conception (percentage of inseminated pregnant; P = 0.23) rates were not different among the 3 treatments. Cows treated between Days 55 and 90 had a higher rate (P < 0.05) of detected estrus during this period (69% for GnRH + PG [Day 6], 70% for hCG + PGF[Day 9] and 72% for hCG + PGF[Day 6]) compared with cows treated between Days 91 and 135 (52% for GnRH + PGF[Day 6], 50% for hCG + PGF[Day 9] and 57% for hCG + PGF[Day 6]). Efficacy of treatment was higher (P < 0.05) in animals treated between Days 55 and 90 (54% for GnRH + PGF[Day 6], 56% for hCG + PGF[Day 9] and 63% for hCG + PGF [Day 6]) compared to animals treated between Days 91 and 135 (36% for GnRH + PGF[Day 6], 35% for hCG + PGF[Day 9] and 47% for hCG + PGF[Day 6]). There were no significant differences in conception between Days 51 and 90 and Days 91 and 135. The interval between parturition-first AI with conception was significantly (P < 0.001) shorter in GnRH + PGF (Day 6; 106 d), hCG + PGF (Day 9; 109 d) and hCG + PGF (Day 6; 103 d) treated cattle than in 106 untreated animals (136 d). Thus, GnRH plus PGF2 alpha or hCG plus PGF2 alpha treatments elicited similar effects in estrus synchronization, treatment efficacy, and conception rate in post-partum dairy cows.