The effect of simultaneous intratesticular injection of the opiate receptor antagonist naloxone and the neurotoxic drug 6-hydroxydopamine (6-OHDA) on testicular growth, compensatory testicular hypertrophy, serum testosterone level and basal testosterone secretion in vitro was studied in neonatal rats. In animals with two testes unilateral intratesticular administration of naloxone alone enhanced, while 6-OHDA alone decreased the weight of the treated gonad. In animals treated simultaneously with these two agents the decrease in testicular weight induced by 6-OHDA was partially prevented by naloxone. In hemicastrated animals intratesticular treatment with naloxone enhanced the extent of compensatory testicular hypertrophy. Treatment of the remaining testis with 6-OHDA + naloxone did not interfere with the diminished compensatory testicular hypertrophy observed following 6-OHDA treatment. Data indicate that naloxone can counteract the degenerative effect of 6-OHDA in animals with two testes but not in hemicastrates.