['To investigate the effects of androstenedione on ovarian follicle development.', 'Experimental study.', 'University research laboratory.', 'Female Wistar-Imamichi rats and BDF1 mice.', 'Rats were injected with androstenedione. Ovarian follicles of mice were cultured in the presence of androstenedione.', 'Ovarian morphology; ovarian cell types undergoing apoptosis; ovarian expression of cytochrome P450 aromatase (P450arom), cytochrome P450 side-chain cleavage (P450scc), and cyclin-dependent kinase inhibitor p27(kip1); serum levels of T, E(2), and P in rats; and ultrastructure of granulosa cells from cultured follicles of mice.', 'In androstenedione-treated rat ovaries, follicular cysts were formed, and apoptotic cells were found in the inner part of granulosa cell layers of antral follicles. Androstenedione administration down-regulated expression of P450arom but up-regulated expression of P450scc and p27(Kip1) in the granulosa cells of antral follicles. Serum T levels were significantly increased in androstenedione-treated rats. In mouse follicles exposed to androstenedione, the granulosa cells contained abundant lipid droplets and mitochondria with complex tubular cristae.'] Does exogenous androstenedione induce formation of follicular cysts and premature luteinization of granulosa cells in the ovary?