Exposure to bulls seem to stimulate cyclic activity in postpartum beef cows, hasten puberty and increase pregnancy rate in heifers. The aim of this experiment was to determine if short-term exposure of pubertal beef heifers to testosterone-treated steers prior to late autumn-early winter bull breeding would increase pregnancy rates. An additional objective was to determine if body weight at the beginning of biostimulation affects the response to the testosteronetreated steers. Hereford heifers (n = 246, 17-22 months of age and 240 to 360 kg) were isolated from bull contact 30 days before the onset of the experiment and assigned to two experimental groups. One hundred and twenty seven heifers were joined with nine testosteronetreated steers (Days 0-15), whereas the other 119 heifers remained isolated from any contact with males. On Day 15, all heifers were joined with 10 bulls for 30 days (Day 45). Pregnancy was determined on Days 60 and 75. On Day 60, no differences were observed between groups in the frequency of pregnant heifers (18.2 vs 7.7% pregnant). On Day 75, more heifers in the biostimulation group that had an initial body weight ≥310 kg were pregnant (61.1 vs 32.4%, for stimulated and non-stimulated heifers respectively, P = 0.016), whereas there were no significant differences in heifers with lower initial body weight (17.9 vs 30.3% pregnant, for stimulated and non-stimulated heifers respectively). In conclusion, pregnancy rate was increased in high body weight heifers during late autumn-early winter natural service following pre-breeding exposure of the heifers to testosterone-treated steers.