In previous studies, administration of adrenocorticotrophin (ACTH; 0.5 mg i.m. b.d. for 5 days) to normal subjects produced an adrenally dependent rise in blood pressure (BP) of some 20 mmHg, accompanied by an increase in cardiac output and an increase in plasma volume. The BP and metabolic effects of ACTH (increase in plasma glucose, fall in eosinophils, increase in body weight and urine sodium retention) were reproduced by infusion of the glucocorticoid (GC) cortisol at rates (6-8 mg/h) which reproduced the blood concentrations of the steroid achieved with ACTH administration. Oral administration (hydrocortisone 200 mg daily) produced similar changes qualitatively, although the cortisol concentrations and increase in pressure (12 mmHg) were less. Plasma volume was increased. To determine the role of urine sodium retention and plasma volume expansion in the hypertension, we gave synthetic steroids to six normal subjects for 5 days, at doses which were calculated to be similar for GC activity, but which had little or no mineralocorticoid (MC) activity. Prednisolone (40 mg/day), methylprednisolone (32 mg/day), triamcinolone (40 mg/day) and dexamethasone (8 mg/day) all produced equivalent GC effects (increase in plasma glucose, increase in total white cell count, fall in direct eosinophil count). There were no MC effects with any of the steroids. Body weight did not increase and urinary sodium excretion increased rather than decreased. Plasma volume (125I human serum albumin) and haematocrit were unchanged. BP rose with all four steroids: systolic BP rose by 13 mmHg with prednisolone, by 9 mmHg with methylprednisolone, by 10 mmHg with triamcinolone, and by 6 mmHg with dexamethasone. Diastolic BP increases were 8, 11, 8 and 7 mmHg, respectively. Thus, neither MC activity nor an increase in plasma volume is essential for steroids to induce an increase in blood pressure. Therefore, screening of synthetic GCs to minimize MC activity will not prevent hypertensive complications.