Serum levels of testosterone, LH and FSH were measured by radio-immunoassay in 215 male out-patients aged 15-83 years. Those with endocrine disorders, liver diseases or neoplasms were excluded. Blood was drawn by venous puncture between 8 and 10 a.m. The results clearly demonstrate, that in males after the age of 50 years, a clear cut decrease of serum testosterone occurs (aged above 60 years: mean value 2.4 ng/ml; aged 20-30 years: mean value 3.5 ng/ml). Serum levels of LH are significantly higher when compared with younger men (9.5 to 5.3 mU/ml). There are also higher levels of FSH (14.5 to 11.1 mU/ml). The increase of LH (and perhaps of FSH) with increasing age points to the conditions in adolescence, where plasma levels of LH rise likewise. This increase, however, signals the maturation of the hypophyseal-gonadal axis, whereas in senility it demonstrates the gradual failure of the balanced interaction and is the hormonal correlate for the so called "male climacterium".