In 3 experiments, 168 ewes of Manchega (n = 72), Churra (n = 62), and Merina (n = 34) breeds were used to test the hypothesis that administration of pure LH, coincident with progestogen removal during superovulation with FSH, causes an increase in the ovulation rate and number of embryos. This administration of LH can further interact with genotype, resulting in breed differential response. In each experiment, the animals were randomly assigned to 1 of 3 treatments. Estrus in all sheep was synchronized with intravaginal sponges of 30 mg of FGA for 12 d, then 270 microg of FSH were administered in 6 injections at 12-h intervals in decreasing doses, starting 48 h before sponge removal. The FSH/LH ratio of the original preparation was 3, and remained constant throughout the treatment in the control group (C). In Treatment 1, (T1) and Treatment 2, (T2), pure LH was administered coincident with progestogen removal-5th FSH injection, and with the 6th FSH injection, at 2 dose levels: 60 and 120 microg, (T1), and 120 and 240 microg (T2). Mating occurred 36 and 48 h after the progestogen removal, and the embryos were surgically collected and morphologically evaluated on Days 7 and 8 after sponge withdrawal. Overall, the results showed that LH administration at the end of the FSH treatment did not increase the ovulation rate and number of embryos in Merino (5.9 +/- 1.4 and 5.6 +/- 1.4, respectively, T1; 7.0 +/- 1.0 and 5.7 +/- 1.2, T 2; 4.9 +/- 1.1 and 2.6 +/- 0.7, C), Churra (6.8 +/-1.4 and 5.2 +/- 1.4, T1; 8.1 +/- 1.5 and 6.3 +/- 1.4, T2; 6.1 +/- 1.5 and 5.4 +/- 1.3, C) and Manchega (6.0 +/- 1.0 and 4.4 +/- 1.0, T1; 5.0 +/- 0.8 and 4.2 +/- 0.8, T2; 4.8 +/- 1.5 and 3.8 +/- 1.0, C). Administration of LH induced a significant (P < 0.05) increase in the frequency of multiple ovulations (72.3 +/- 4.3 %, T1; 74.1 +/- 11.5 %, T2; 55.6 +/- 5.9 %, C) paralleled to a decrease in the occurrence of ewes with no ovulations (8.7 +/- 2.6 %, T1; 7.6 +/- 4.6 % T2; 17.3 +/- 3.2 %, C) or 1 to 2 ovulations (18.7 +/- 4.6 %, T1; 18.1 +/- 7.5 %, T2; 26.8 +/- 5.8 % C), regardless of breed or dose of LH. No increase in the mean number of viable embryos was observed, probably due to both the high individual variability and the lower fertilization rates observed in sheep showing multiple ovulations.