Determinations were made of testes and seminal vesicle weights as well as plasma testosterone levels in aging male mice of inbred strains DBA/2J and C57BL/6J. Results indicate that plasma testosterone levels are significantly higher in C57BL/6J mice than in DBA/2J, although at 2 months of age the latter strain exhibited significantly higher levels than the former. Of significant interest is the finding that both strains did not exhibit any significant alteration in testosterone levels during their adult life except at 2 months of age. Seminal vesicle weights differed significantly between the two strains only at 20 and 28 months. Suggestion is further made that body weight correction for weight of organs, such as testes and seminal vesicles, may be inappropriate; rather, the most appropriate adjustment of these organ weights to correct for correlations with body weight may be covariance analysis.