Serum 5alpha-dihydrotestosterone (DHT) and testosterone (T) were measured in 98 normal adult men aged between 20-80 years, separating T and DHT by thin layer chromatography and using a sensitive and reliable radio-immunoassay. In three age groups between 20-40, 40-60 and 60-80, the means +/- SEM for serum DHT were 84 +/- 4, 79 +/- 3 and 67 +/- 3 (ng/100 ml) respectively. The corresponding values for testosterone were 559 +/- 25, 491 +/- 25 and 475 +/- 28 (ng/100 ml). A significant decrease was observed in the DHT level of the 60-80 years age group compared to either the 20-40 (P less than 0.01) or the 40-60 (P less than 0.02) age groups. There was a moderate decline in the testosterone level of the 60-80 years age group compared to 20-40 years (P less than 0.05) but there were no significant changes in the testosterone levels between other groups.