We have investigated on eventual relationships existing in men between plasma levels of 17-a-OH-Progesterone (17-a-OH-P) and, plasma levels of Testosterone (T) and on the modifications of these relationships after stimulation of the testicles by HCG (Human Chorionic Gonadotropin). An inverse correlation exists between basal plasma levels of the two steroids and their delta max (%max increase 96 h after the injection of 5000 I.U. of hCG): r = -06, p 0.01. This suggests that the enzymatic steps of the delta -4 steroidogenic pathway are rate-limiting for the synthesis of T. The delta max of the two-steroids are also inversely correlated with the circulating levels of LH (Luteinizing Hormone) (r = -05, -06; p less than 0.01) suggesting that the rate-limiting activity of the delta 4 pathway is under endogenous LH control. A desensitized state of this pathway can be expected in presence of high circulating levels of LH. Similarly to LH also the FSH plasma levels, an index of function of the seminiferous tubules, are inversely correlated with the 17-a-OH-P and T delta max (r = -0.4, -05; p 0.005) indicating close relationships between tubular and interstitial functions. In men with idiopathic oligozoospermia and high circulating levels of FSH we have found increased 17-a-OH-P/T ratios after hCG.