The mechanism of cortisol-induced hypertension remains unknown. We investigated a possible role of erythropoietin (EPO) as a mediator of hypertension in healthy male subjects treated with cortisol. In Study 1, blood pressure (BP) and serum EPO concentrations were measured on alternate days in nine subjects treated with 80 mg of cortisol per day for 5 days. In Study 2 the same parameters were measured in eight subjects randomised to cortisol (80 mg/day) or placebo and 10 subjects randomised to cortisol (200 mg/day) or placebo for 5 days. In Study 1, cortisol caused a significant increase in systolic BP (SBP) (115 +/- 2 vs 126 +/- 2 mm Hg, control vs day 5, P < 0.001) and serum EPO concentrations (14.5 +/- 2.7 vs 24.3 +/- 2.7 mU/mL, P < 0.001). In Study 2 both doses of cortisol increased SBP (118 +/- 2 vs 113 +/- 2 mm Hg, 80 mg cortisol vs placebo, P < 0.05 and 129 +/- 3 vs 113 +/- 2 mm Hg, 200 mg cortisol vs placebo, P < 0.001). Serum EPO concentrations were significantly increased at 200 mg cortisol (25.2 +/- 11.9 vs 15.9 +/- 3.5 mU/mL, P < 0.01) but not 80 mg cortisol (21.3 +/- 2.9 vs 14.9 +/- 3.1 mU/mL). In the 200 mg group there was a positive correlation between the change in SBP and the change in serum EPO concentration (r2 = 0.43, P < 0.05). These results point to a possible role for EPO as the mediator of cortisol-induced hypertension. Journal of Human Hypertension (2000) 14, 195-198.