The aim of the present study was to evaluate the effects of indole-3-acetic acid (IAA) on survival, activation, and growth of caprine preantral follicles using histological and ultrastructural criteria. Pieces of caprine ovarian cortex were cultured for 1 or 5 days in Minimum Essential Medium (MEM - control medium) supplemented with different concentrations of IAA (10, 20, 40, or 100 ng/ml). Small fragments from noncultured ovarian tissue and from those cultured for 1 or 5 days in a specific medium were processed for classical histology and ultrastructural analysis. Follicle and oocyte growth were evaluated before and after 1 or 5 days of culture in the various media tested. The results showed a greater percentage of histologically-normal follicles in MEM alone or MEM supplemented with IAA (20 ng/ml) than other treatments. Indole-3-acetic acid added at a concentration of 20 or 40 ng/ml increased the proportion of primordial follicles that entered the growth phase after 5 days. In the presence of 10 or 20 ng/ml IAA, the follicles increased in diameter after 5 days of culture. Ultrastructural studies, however, did not confirm maintenance of the morphological integrity of caprine follicles cultured for 1 or 5 days in MEM supplemented with 20 ng/ml of IAA as was demonstrated by histological studies but in contrast showed distinct degenerative changes in both oocytes and granulosa cells. In conclusion, ultrastructural integrity of caprine preantral follicles can be successfully maintained after in vitro culture for 5 days in MEM without addition of IAA. In addition, ultrastructural analysis is necessary to judge the morphology of cultured preantral follicles since IAA negatively affects the ultrastructural composition of in vitro activated follicles, which was not detected by classical histology.