The present study was designed to investigate the effect of haloperidol on plasma corticosteroid levels in a small sample of unmediated psychiatric patients requiring acute care. Seven young male normotensive subjects in metabolic balance received a single dose of haloperidol (2 mg i.v.). Blood samples were collected for the radioimmunoassay of plasma renin activity, cortisol and aldosterone concentration at baseline and 3, 6, 12 and 24 h after injection. In five out of seven patients a significant, transient elevation of plasma aldosterone level was observed within 12 h from administration. In contrast, plasma renin activity and cortisol concentration were unchanged. Possible clinical implications of the neuroleptic-associated aldosterone elevations are discussed.