5-Bromo-2'-deoxyuridine (BrdU) has been recently found to enhance adrenal glucocorticoid secretion in rats. Since BrdU-labeling is today the most employed technique for assessing cell proliferation, we have investigated whether this side-effect of BrdU may influence mitotic activity (as evaluated by metaphase-arrested cell counting) in the thymus cortex and regenerating adrenal cortex. BrdU (1 mg/100 g body weight) evoked a 6-fold rise in the blood level of corticosterone in intact rats, but not in animals bearing regenerating adrenals, in which glucocorticoid secretion was very low. Mitotic index in the thymus of intact rats was not changed by BrdU, but it was strongly depressed in regenerating adrenals. These findings rule out the possibility that the glucocorticoid secretagogue action of BrdU exerts a sizable effect on the proliferative activity of both thymus and adrenal cortex. The marked lowering in the mitotic rate in the regenerating adrenal cortices could be ascribed to the well-know differentiation promoting action of BrdU, a finding which, if confirmed, would make highly unreliable the use of BrdU-labeling technique in cultures of adrenocortical cells.