The effect of PRL supplementation in lithium-treated rats on spermatogenesis, testicular delta 5-3 beta-hydroxysteroid dehydrogenase and 17 beta-hydroxysteroid dehydrogenase activities, and serum levels of FSH, LH, PRL and testosterone were studied on the 22nd day of the experiment. Subcutaneous injections of lithium chloride at a dose of 2.0 mg.kg-1.day-1 for 21 days resulted in a significant inhibition of spermatogenesis at stage VII of the seminiferous epithelial cycle, along with remarkable diminution of serum levels of the above hormones and suppression of the activities of the above two testicular steroidogenic enzymes. Administration of bovine PRL at a dose of 0.25 mg.kg-1.day-1 plus lithium treatment resulted in a remarkable protection of spermatogenic and steroidogenic activities of the testes, along with restoration of serum levels of FSH and testosterone. It is concluded that PRL can markedly protect the testicular dysfunction induced by lithium chloride treatment in rats.