To investigate the secretion of renin from the Leydig cells of human testis, plasma renin activity (PRA) in left internal spermatic vein (ISV) and cubital vein (CV) was measured at the time of surgical repair of varicocele in 30 patients between 26 and 39 years old. Sixteen of them were given a single i.m. injection of hCG (10,000 iu) 4 days before operation, whereas the remaining fourteen were not treated. Although mean PRA levels of CV in treated and non-treated groups were similar (1.35 +/- 0.46 and 1.19 +/- 0.59 ng/ml/hr, respectively), mean PRA level of ISV in the treated group (2.85 +/- 1.15 ng/ml/hr) was significantly higher than that of CV in the same group (1.35 +/- 0.46 ng/ml/hr), and that of ISV in the non-treated group (1.39 +/- 0.67 ng/ml/hr) (both P less than 0.01). Mean testosterone level of the same ISV was clearly higher in the treated than in non-treated group (P less than 0.005). These data seem to suggest the secretion of hCG-induced renin from the human testis in vivo and the possibility of relation between androgens and secretion of renin.