Efficiency in reference to pregnancy rates of breeding beef bulls with estrus synchronized cows and heifers was tested. Most bulls (104 of 112) were given a breeding soundness examination and two 10-min libido/serving capacity tests. Females received either Syncro-Mate-B (SMB) or two injections of Prostaglandin F 2α (PGF) to synchronize estrus. They were assigned to single-sire breeding groups with bull-to-female ratios ranging from 1:7 to 1:51. Control groups consisted of untreated females maintained in single-sire breeding pastures with ratios from 1:24 to 1:37. Continuous observations of sexual activity were made for 30 h (SMB) and 48 h (PGF). After the 120-h posttreatment breeding period, females were placed in breeding pastures. During the synchronized breeding period the percentage of pregnant cattle of total treated was 43.5 ± 1.7% compared (P < 0.01) with 58.9 ± 3.3% for the control group after 23 d of breeding. At end of 28-d (treated) and 46-d (control) period, the percentage of pregnant females was 75.0 ± 2.4 and 79.6 ± 4.7, respectively (P > 0.05). In SMB trials, the percentages of females exhibiting estrus, those serviced at estrus and those pregnant following service during the synchronized breeding period were 90.8 ± 1.5, 73.3 ± 4.5 and 56.4 ± 5.6%, respectively. In PGF trials, the means for these same factors were 78.3 ± 2.4, 70.4 ± 5.9 and 56.1 ± 6.5%, respectively.