A study was designed to compare the effects of exogenous and endogenous oestrogens upon the expression of steroid 17α-hydroxylase/C17,20-lyase (CYP17) in the immature hypophysectomized rat ovary. C17,20-lyase activity was measured ex vivo using [21-14C]progesterone, while serum concentrations of androstenedione indicated in vivo activity. Immunocytochemistry was used to localize the enzyme within the ovary. Activity of CYP17 increased dramatically and remained high in thecal and interstitial cells after injection of equine chorionic gonadotrophin, even though serum oestradiol (OE2) levels exceeded 2 nmol/l. Production of comparable serum levels by the use of Silastic capsules containing OE2 greatly suppressed but did not stop, expression of the enzyme induced by repeated doses of human chorionic gonadotrophin (hCG). Subcutaneous implantation of Silastic capsules (1 cm) containing diethylstilboestrol (DES) stimulated follicular growth and increased ovarian weight by 67%, 5 days later. injection of 50 IU hCG at various times after removal of the implant produced time-dependent changes in CYP17 activity and serum androstenedione levels, when measured 30 h later. Although the initial effect of removal of DES was an increase in CYP17 activity, delaying injection of hCG resulted in a reduced response. The results indicated that: (1) endogenous oestrogens do not inhibit CYP17 expression; (2) exogenous oestrogens only reduce the number of thecal/interstitial cells expressing CYP17 when they are exposed to hCG; (3) pretreatment with oestrogen removes the ovarian interstitial but not the thecal cell expression of CYP17 in response to hCG; and (4) oestrogens can be stimulatory for CYP17 expression in thecal cells.