Sex hormone-binding globulin (SHBG), testosterone (T), 5 alpha-dihydrotestosterone (DHT) and 17 beta-oestradiol (E2) were measured in 93 men in the age range 36--80 yr. Assuming that the binding of the steroids to blood albumin and SHBG follows the law of mass action, non-SHBG bound steroid concentrations (Tn, DHTn, E2n) were calculated from the SHBG and the respective total hormone values. The main results were: (1) Mean Tn, DHTn, E2n and SHBG remained on the constant level until the age of 65 yr. In men 66--80 yr old Tn and DHTn were found to be significantly lower (P less than 0.001), whereas E2n and SHBG were elevated (P less than 0.05, P less than 0.001, respectively) when compared with men age 36--55 yr. (2) In contrast to total T and DHT, which correlated positively with SHBG (P less than 0.001) in Group I (age 36--55 yr) and also when eliminating the influence of age (P less than 0.05, P less than 0.01, P less than 0.01 for T, DHT and E2) in group II (age 56--80 yr), the correlation between Tn and SHBG was insignificant, whereas a negative correlation between DHTn and SHBG (P less than 0.01) was present in Group I. (3) The age dependent increase of SHBG in Group II can be best explained by the respective alterations of the E2n/DHTn ratio. These results are discussed with respect to the relation between the SHBG, the total steroid and the non-SHBG bound steroid concentrations.