The study aimed to examine the effects of pinealectomy (PX) and melatonin on adrenal cortex of orchiectomized rats and to explain whether the testosterone action on adrenal cortex is modified by the pineal gland. Neither PX (53 days) nor melatonin administration (10 days) had an effect on the structure as well as on the studied functional parameters of the adrenal gland--corticosterone output by homogenates, 11 beta-hydroxylase and 5 alpha-reductase activities, liver corticosterone metabolism and serum corticosterone level. On the contrary, PX of 14 months duration resulted in an increase in 5 alpha-reductase activity. Testosterone administration into orchiectomized rats lowered corticosterone output and liver corticosterone metabolism and these effects were unaffected by PX or melatonin. Testosterone lowered 5 alpha-reductase and 11 beta-hydroxylase activities, the effects not modified by PX, however, after melatonin administration activities of these enzymes were similar as in orchiectomized rats. Obtained results cast doubts on a physiological role of the pineal gland and its principal hormone--melatonin--in regulation of adrenocortical structure and secretory activity and suggest that testosterone action on the rat adrenal cortex is not modified by the pineal gland.