The diterpene forskolin was found to activate the adenylate cyclase system in intact tissue and membrane preparations of the immature rat ovary. The cyclic AMP (cAMP) response reached a maximal level after 5 min and no decline was observed even after 4 h of incubation. Forskolin stimulated production of both progesterone and testosterone in a pattern similar to that produced by luteinizing hormone (LH) or dibutyryl-cAMP (dbcAMP). In combination with LH, follicle-stimulating hormone (FSH) or prostaglandin E2 (PGE2), forskolin potentiated the hormone effects on adenylate cyclase activity in membrane preparations. Pretreatment with LH or PGE2 desensitized the cells to further hormone stimulation, while the forskolin response was unaffected. Pre-exposure to forskolin did not desensitize the cells to a subsequent stimulation by LH or PGE2. The presence of 8-bromo-cAMP (brcAMP) in the preincubation medium reduced the subsequent hormone response. These results demonstrate a rapid and sustained activation of the adenylate cyclase system by forskolin in the rat ovary. The steroidogenic response was similar to that of known stimulators of ovarian cells (LH, dbcAMP). The inability of forskolin to induce desensitization of the adenylate cyclase system demonstrates, however, important differences between hormone and non-hormone activation. Consequently, forskolin can be a useful tool for investigation of the mechanisms involved in the desensitization process.