Preantral follicles (140–160 μm) were isolated mechanically from the ovaries of 10-day-old rats and cultured in groups of four to six for 6 days in medium containing 0, 1, 10, or 100 ng/ml of activin A. Activin stimulated (P < 0.05) the growth of preantral follicles in a dose-dependent fashion and enhanced the proliferation of preantral, oocyte-free follicular cells. Furthermore, treatment with activin induced the majority of follicles to form an antrum-like structure and helped to maintain the ultrastructure of these follicles during culture. Activin A also induced further changes characteristic of follicle and oocyte maturation, such as the elongation of granulosa cells contacting the oocyte and migration of the cortical granules to the oocyte cortex. In addition, gene expression for activin and activin receptor type II (ActR II) was demonstrated in both the oocytes and the somatic cells using the reverse transcription-polymerase chain reaction, and immunohistochemical studies demonstrated the presence of activin and ActR II proteins in the somatic tissue and, especially, the oocytes of these follicles. It is concluded that, in vitro, activin A stimulates the growth of rat preantral follicles and promotes antrum formation. Furthermore, because activin A and ActR II are synthesized within preantral follicles, intrafollicular activin likely plays an important role in early follicular development.