The steroidogenic potentials of testes from 6-mo-old and approximately 28-mo-old CB6F1 mice were compared by measuring the secretion rate of 11 steroids produced by testes perfused in vitro. Testes from 6- and 28- to 30-mo-old mice secreted similar amounts of testosterone, but marked differences in the secretion of testosterone biosynthetic intermediates were discovered. Testes of old mice produced significantly more (P less than 0.01) delta 5-steroids than young male mouse testes. The presence or absence of sexual activity among old male mice was not correlated with changes in testicular steroidogenesis or plasma steroid levels. The results suggest that selected alterations in androgen biosynthesis occur during aging, but that the decline in reproductive performance among old mice is not attributable to deficits in testicular steroidogenesis. Rather, age-associated losses in sexual activity may be mediated by disturbances in the central nervous system.