Tritiated testosterone and [14C]dihydrotestosterone (DHT) were administered by constant iv infusion into five young and five elderly men undergoing diagnostic cardiac catheterization. The radioactivity concentrations of free and conjugated DHT in arterial and hepatic vein blood samples were then determined. The analysis of the 3H:14C ratio of free DHT in arterial and hepatic vein blood showed that in both groups, the 3H:14C ratio of free DHT was the same in arterial and hepatic vein blood, indicating that splanchnic tissue is not the source of blood DHT from testosterone. This is in agreement with data that the transfer constant across the liver ([rho]T-DHT SD) was undetectable. In both young and elderly men, a significant increase of the 3H concentration of DHT glucuronide in hepatic vein blood was observed, indicating that the splanchnic compartment could be the site of production of DHT glucuronide. The 3H:14C ratio of DHT glucuronide was much higher than that of free DHT in both groups, suggesting that DHT glucuronide is derived from the blood testosterone pool, and most of the DHT from testosterone seems to be conjugated before mixing with blood DHT. This study indicates that a large fraction of DHT produced in the liver from testosterone is efficiently conjugated or further metabolized, and this results in the lack of splanchnic production of free DHT in men.