Lines of mice have been divergently selected on one of two traits: (i) estimated fat content at 14 weeks of age, which has resulted in a 5-fold divergence, and (ii) body weight at 10 weeks of age, which has resulted in a 3-fold divergence. Individuals from each line were castrated or sham operated at 10 days of age and subsequently given either exogenous testosterone or the appropriate control from 14 days of age. Castration increased fat content and decreased lean weight in all lines, an effect which was not reversed by administration of testosterone. Body weight was reduced by around 10% as a result of castration and this effect was at least partially reversed by exogenous testosterone. Analysis of body weight, fat content and lean mass at 10 weeks of age failed to detect any interaction between these treatments and genetic background. It is therefore concluded that testosterone metabolism has not contributed disproportionately to the response to artificial selection in spite of its known effects on growth and body composition.