1. In one experiment [7α-3H]pregnenolone was infused continuously for 12min into the left spermatic artery of a sexually mature boar and blood was collected during this period by continuous drainage from the spermatic vein. After infusion, the testis was removed and immediately cooled to −196°C. 2. From both the testicular tissue and the spermatic venous plasma, 3H-labelled 16-unsaturated C19 steroids were isolated and characterized and their radiochemical purity was established. 5α-Androst-16-en-3α- and 3β-ol occurred mainly as sulphate conjugates and to a lesser extent as free steroids. Only traces of these alcohols occurred as glucosiduronate conjugates. 5α-Androst-16-en-3-one was found in the free (ether-extractable) fraction. 3. The isotope concentration of each of the 3H-labelled 16-unsaturated C19 steroids in testicular tissue was different from that in spermatic venous plasma. 4. The ratios of tritiated 5α-androst-16-en-3α- and 3β-ol (free steroids) to their respective sulphate conjugates in the testicular tissue were less than the ratios of the same compounds in the spermatic venous plasma. The possibility that the sulphates are partially hydrolysed by testicular sulphatases before secretion is discussed. 5. In a second experiment, a continuous close-arterial infusion of [7α-3H]pregnenolone into the left testis was performed over a 200min period and all the urine that accumulated during the infusion was collected for analysis. 6. No 3H-labelled 16-unsaturated C19 steroids were detected in the urine as free steroids. Only a trace of 5α-androst-16-en-3α-ol was detected conjugated as glucosiduronate, whereas the corresponding 3β-alcohol occurred mainly as glucosiduronate and to a lesser extent as sulphate. 7. The absence of 5α-androst-16-en-3β-ol glucosiduronate in the spermatic venous blood and its presence in considerable amount in the urine may be attributed to hepatic glucuronyl transferase activity.

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