In normal and obese young males [90-120% and > 160% of ideal body weight (IBW); IBW = 100%], plasma concentrations of testosterone [T], androstenedione [A], estrone [E] and estradiol [E2] were measured. Metabolic clearance [MCR] and production rates of androstenedione and the conversion ratios of androstenedione to testosterone, estrone and estradiol were determined using the constant infusion technique. In the obese subjects, IBW was inversely correlated (P < 0.001) with plasma concentrations of A (r = 0.81) and T (r = 0.87); the levels of E (r = 0.92) and E2 (r = 0.95) increased with IBW (P < 0.001). When normal and obese subjects were compared as groups, plasma A decreased 1.24 .+-. 0.13-0.93 .+-. 0.15 ng/ml (mean .+-. SD) and plasma T decreased 5.89 .+-. 0.82-3.29 .+-. 0.92 ng/ml (P < 0.001); E increased 28.2 .+-. 3.4-60.0 .+-. 9.4 pg/ml and E2 increased 21.7 .+-. 3.5-43.9 .+-. 5.3 pg/ml. The T to A and the E2 to E ratios were not different in obesity but changes in IBW were positively correlated (P < 0.001) with differences in the E to A (r = 0.93) and E2 to T ratios (r = 0.93), indicating that fat tissue may aromatize androgens, whereas reduction of 17-oxosteroids appears to be of minor importance. As the MCR of A increased with IBW (from 2156-2636 l/day, P < 0.05), while plasma levels decreased, the apparent production rate of A was not influenced by the degree of obesity. The conversion of A to E (r = 0.89) and of A to E2 (r = 0.82) was enhanced in obese subjects (P < 0.001). Apparently, enhanced aromatization of androstenedione due to an increased adipose tissue mass may account for the high plasma estrogen levels observed in obese men.