The effect of testosterone on compensatory renal hypertrophy (CRH) remains controversial. We therefore examined the effect of exogenous testosterone on CRH in adult male and female rats after unilateral nephrectomy. The influence of endogenous testosterone was studied by comparing the degree of CRH in normal male, castrated male, and in testosterone receptor deficient male pseudohermaphrodite rats. Furthermore, serial determinations of serum testosterone levels were performed after unilateral nephrectomy in male rats. Compensatory renal hypertrophy was comparable between male rats--with or without exogenous testosterone administration--and between normal male, castrated male, and psuedohermaphrodite male rats. In contrast, exogenous testosterone administration in female rats enhanced CRH. Serum testosterone levels fell markedly after unilateral nephrectomy or sham surgery, but increased to 183% and 234% of control values at 1 and 2 days after surgery in the unilaterally nephrectomized rats. At no time, however, did they exceed the range of normal values. The results indicate a different effect of testosterone on CRH in male and female rats.