The effects of a single im injection of 10,000 IU of hCG on testicular steroid secretions were studied in 15 normal men and 54 patients with varicocele. Plasma levels of the following steroids were determined by radioimmunoassay: T, DHT, A, DHA, 5Adiol, E1, E2, 5P, 170H5P, 170HP, E1S, E2S, TE1, TE2, DHAS, 5AdiolS and hCG as well as FSH and LH in baseline samples. In normal men, peripheral venous blood was collected at 0, 1, 2, 4, 6, 8, 24, 36, 48, 72 and 96 h after hCG. Three types of steroid response were observed. The first, biphasic with an early peak within 2 h and a delayed one after 24 h, was shown by T, its unconjugated precursors and DHT. The second with an early rise plateauing for 6 h and followed by a 36 h peak characterized the estrogen pattern. Finally DHAS did not demonstrate a significant increase but 5AdiolS fluctuated at higher levels than the baseline ones. A significant correlation was only observed between hCG maximum value and E2 and TE1 maximum relative responses. In the patient group, peripheral blood was collected at 0, 1, 1.5, 2, 4, 6, 8, 10, 24 and 48 h after hCG. Baseline levels of all steroids were similar in the two groups. After hCG the same delayed pattern was observed as in the controls. The early relative response was low for T while A, DHT, E2 and E1S levels decreased. Conversely 170HP was higher until 24 h. Spermatic vein blood was collected before and at 1.5, 24 and 48 h after hCG. Steroid baseline levels in the varicocele side were similar to the other. After hCG, despite very large variations, general steroid patterns were comparable to those observed in peripheral vein blood. The important fall at 24 h of T/170HP and T/170H5P concomitant with that of T/E2 might be related to an inhibition of C17-20 lyase activity. The testicular secretion of all the measured steroids, including E1S, could be demonstrated except for DHAS. From our findings, a biogenetic defect evidenced by the fast response to hCG administration might exist in varicocele patients. In view of these results, the protocol of the hCG test should be improved using more frequent blood sampling and determining, besides T, its precursors and estrogen conjugates.