Somatostatin has been demonstrated in the testis. In the present investigations the effect of intratesticular injection of somatostatin on serum testosterone level and in vitro basal testosterone secretion of the testis was studied in immature and adult rats. Intratesticular injection of somatostatin in adult rats with two testes in situ decreased serum testosterone concentration and basal testosterone secretion in vitro. Similar treatment in immature animals had no effect on the parameters studied. In immature hemicastrates, however, local administration of the peptide induced a significant rise both in basal testosterone secretion in vitro and serum testosterone level, and resulted in an increase in testicular weight. In adult hemicastrates the peptide did not influence testicular functions. These results indicate that somatostatin might play a modulatory role in testicular steroidogenesis. The data also suggest that the effect of somatostatin is age-dependent, and that hemicastration might modify local action of the peptide.