Alterations in the basal lamina (BL) of developing follicles were studied by immunofluorescent microscopy using antibodies against type IV collagen, laminin, and fibronectin, and by electron microscopy. Ovarian development was induced in immature rats by sequential administration of estradiol, follicle-stimulating hormone (FSH) and human chorionic gonadotropin (hCG). A continuous BL was observed in healthy follicles treated with estradiol and FSH. As determined by immunofluorescence, laminin, type IV collagen, and fibronectin were restricted to the BL and the theca but not to the granulosa. When follicles were allowed to undergo atresia or induced to ovulate with hCG, the BL became fragmented. This was confirmed by electron microscopy of healthy, atretic, and luteinizing follicles which showed that in healthy follicles the BL was continuous, whereas in both atretic and luteinizing follicles, it was fragmented. Atresia was also associated with the penetration of thecal cells into the follicles. These observations indicate that the intact BL present in healthy follicles undergo extensive changes during atresia and ovulation.