Sexually dimorphic differences in voluntary sodium intake, locomotor activity, immobility in the swim test, and aggressive behavior were found to be altered in transgenic CD-1 mice that overexpressed transforming growth factor α (TGFα). In contrast to nontransgenic CD-1 mice, immobility in the swim test was longer and sodium intake higher in the male TGFα mice than in the female TGFα mice. These findings indicate that the male TGFα mice exhibited feminization of some behaviors. Furthermore, the male TGFα mice were highly aggressive. Castration reversed the behavioral effects in the adult male transgenic mice, but ovariectomy did not reverse the behavioral effects in the adult female transgenic mice. Thus the feminizing effect of TGFα on some nonreproductive behaviors and increased aggressive behavior in male mice may be mediated through an interaction between this growth factor and gonadal hormones.