The influence of testosterone on purine nucleotide metabolism in rat kidney has been investigated in adult and in prepubertal castrated rats. Results have been evaluated through biomathematical model. Castration enhanced the turnover of purine nucleotides in adult rats and reduced it in young castrated rats. Treatment with testosterone in the castrated rats further enhanced nucleotide turnover both in the adult rats and also in the second group, with an oscillatory profile. A clear effect on the inosinic branch point was demonstrated, and specifically on GMP formation, which was opposite according to the age of the animal. The different behavior in the two groups after castration was partially ascribed to the action of other hormones in the absence of testosterone. The observed changes show that the action of the hormone is not limited to sexual organs; they might be at the basis of variations in cellular size and number which probably occur in the kidney after orchiectomy and following androgen administration.