Two breeds of prepubertal gilts known to differ in their ovarian development were used to compare the effect of hormonally stimulated ovulation on follicle numbers and proportion of follicular atresia in size classes other than preovulatory follicles. Hormonal treatments (hCG or eCG-hCG) were given to Meishan gilts (n = 36) at 3, 2, and 1 standard deviations (SD) before the mean age of puberty (51, 64, and 77 days of age) and to Large White gilts (n = 24) at 3 and 1 SD before the mean age of puberty (140 and 166 days of age). Ovarian follicle populations determined in the right ovaries 18 h after hCG injection alone (hCG) or 72 h after eCG injection (eCG-hCG) were compared with those in the left ovaries, which had been removed prior to injections. Follicles with dispersed cumulus cells around the oocytes (expected ovulations) observed after hormonal treatments (right ovaries) were considered for follicular measures and categorized into specific size classes according to their diameter. Human CG alone did not affect any follicular parameters measured at any age in either of the two breeds studied. The overall population of nonatretic follicles was significantly reduced by the eCFG-hCG treatment in the Large White breed when imposed at either 3 or 1 SD before the mean age of puberty (262.6 vs. 158.8; p < 0.01). Among the various follicle size classes studied, eCG-hCG treatment significantly decreased the mean number of follicles in size classes 2 (1.13-2.00 mm in diameter) and 3 (2.01-3.56 mm in diameter) at either 3 or 1 SD before the mean age of puberty in the Large White gilts (p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)