The acute and chronic effects of estradiol (E2) on the serum levels of four delta5,3-beta hydroxysteroids and their four delta4, 3-keto products were studied in four ovariectomized women with and without adrenal stimulation by ACTH. Six hour infusions of saline and of synthetic 1-24 ACTH were administered and later repeated with a two hour infusion of E2 50 mug/h. The patients were then given 50 mug of ethinyl estradiol (EE2) p.o. for 4 to 6 weeks and the control and ACTH infusions were again repeated. Levels of pregnenolone3 (Pe), 17alpha-hydroxypregnenolone (17 Pe), progesterone (Po), 17alpha-hydroxyprogesterone (17 Po), dehydroepiandrosterone (DHEA), androstenedione (Adione), androstenediol (Adiol), and testosterone (T), as well as cortisol and DHEA-sulfate were measured by radioimmunoassay on serum samples taken at 1200 and 1300 h. There was no significant effect of E2 or EE2 in the doses administered with or without exogenous ACTH on 3 betaOHSD activity as reflected in absolute steroid levels or in the ratio of concentrations of each delta5:delta4 steroid pair. During the 4th and 5th hour of ACTH infusion, the plasma level of 17 Pe (mean 22.5-fold stimulation) was most elevated, followed by 17 Po (12.5-fold), Pe (10-fold), cortisol (5.9-fold) and Po (4.5-fold), with smaller increases for the other steroids. These results, as well as the pattern of change in plasma levels in one of the subjects in whom fifteen minute samples were measured, provide further evidence suggesting that the major pathway for cortisol biosynthesis in vivo proceeds from Pe via 17 Pe, and not via Po.