A single dose of corticosterone (1 mg/kg b.w. i.p.) increased the 5-hydroxytryptamine (5-HT) content in the mesencephalon, amygdala and hypothalamus. The maximum was observed at 15 min following administration. The 5-HT level returned to normal between 60 and 180 min. The 5-hydroxyindoleacetic acid (5-HIAA) content decreased in the mesencephalon and hypothalamus at 15 min, and then increased to the maximum at 30 min in the mesencephalon and at 45 min in the hypothalamus and amygdala. There were no changes in the septum and hippocampus either in 5-HT or in 5-HIAA content following corticosterone administration. Desoxycorticosterone administration (1 mg/rat i.p.) was ineffective. Tritiated 5-HT uptake did not change in the hypothalamus and mesencephalon tissue in vitro after corticosterone or desoxycorticosterone treatment. Adrenalectomy caused a decrease in the 5-HT content in all brain areas studies. 5-HIAA decreased only in the hypothalamus. Corticosterone administration normalized the 5-HT and 5-HIAA content in the hypothalamus and mesencephalon in adrenalectomized rats. Tritiated 5-HT uptake was lower in the hypothalamus and mesencephalon in adrenalectomized rats. Corticosterone administration increased the activity back to normal in the hypothalamus, however desoxycorticosterone was ineffective. The data suggest that the plasma corticosterone level plays a role in the regulation of the activity of the serotoninergic system in certain limbic brain structures.