Tissue available (bioavailable) testosterone (T) includes circulating free T (FT) and albumin-bound T. A reasonable assessment of bioavailable T can be made by using 50% ammonium sulfate to precipitate sex hormone-binding globulin (SHBG)-bound T. The supernatant non-SHBG-bound T (non-SHBG-T) correlates well with physiological androgen activity. To assess bioavailable T in normal aging men, we analyzed serum samples from seven healthy aged men (65-83 yr old) and compared the results to samples from 13 young men (22-39 yr old). Mean serum T, FT, and LH concentrations were not significantly different in the 2 groups. However, the mean absolute non-SHBG-T level was significantly lower (P less than 0.005) in the older group. In a separate population of 20 impotent but otherwise healthy men (5 27-37 yr old, 10 48-64 yr old, and 5 66-69 yr old), the mean absolute non-SHBG-T concentration was lower in middle-aged (P less than .01) and elderly men (P less than 0.001) than in young men. The absolute FT was lower only in the elderly group (P less than 0.05), while mean LH and T levels were similar in all 3 age groups. These data suggest that serum concentrations of tissue available T are decreased in aged men and that non-SHBG-T measurement is a more sensitive indicator of this decrease than are serum T or serum FT measurements. These changes appear to begin by middle age.