Serum 5alpha-androstane-3alpha, 17beta-diol was measured in 60 normal men aged 23-80 years using sensitive and reliable radioimmunoassay techniques developed in our laboratory. When these normal men were classified into three age groups of 20-40, 40-60 and 60-80 years the values (mean +/- SEM) for serum 5alpha-androstane-3alpha, 17beta-diol were 857.3 +/- 36.3, 878.1 +/- 33.8 and 645.6 +/- 26.6 pmol/l, respectively. There were significant differences between the level of this steroid in the age group 60-80 when compared to either the 20-40 group (p less than 0.001) or the 40-60 group (p less than 0.001). No significance difference was found between the values for the two younger groups. The decline of this steroid with age corresponds with the diminishing level of testosterone and dihydrotestosterone previously reported from this laboratory.