The role of testosterone in follicular development was investigated by immunizing female rabbits (mean wt 1.4 kg) to testosterone-3-bovine serum albumin (T-3-BSA). Controls received BSA. Follicular diameters and histology, and ovarian and uterine weights were recorded at intervals up to 11 weeks. At 5 weeks T-3-BSA ovaries did not differ from controls in either histology or follicular diameter (number of follicles greater than or equal to 1.0 mm were 12.5 +/- 3.4 and 17.0 +/- 1.0 for BSA and T-3-BSA treated animals respectively). By 8 weeks T-3-BSA animals had multiple cystic and hemorrhagic follicles. T-3-BSA ovaries contained more follicles greater than or equal to 1.0 less than 1.5 mm (27.3 +/- 3.1 vs. 15.3 +/- 2.9, p less than 0.01) and greater than or equal 1.5 mm diameter (5.8 +/- 1.7 vs. 0.4 +/- 0.3, p less than 0.005). At 11 weeks T-3-BSA ovaries contained more follicles less than 1.5 mm in diameter (5.0 +/- 0.9 vs. 0.9 +/- 0.3, p less than 0.001). Increased vascularization, some thecal hypertrophy and marked interstitial cell hypertrophy were characteristic of the T-3-BSA ovaries at 8 and 11 weeks. These results suggest that testosterone has a role in the regulation of follicular development.