1. To gain insight into the mechanism of mineralocorticoid-induced blood pressure rise in man we performed haemodynamic studies in six normotensive volunteer subjects before and during administration of the synthetic steroid 9α-fluorocortisol (0·8 mg daily) for a period of 6 weeks. In a further study, performed in seven subjects, plasma noradrenaline concentration and reactivity to exogenous noradrenaline were determined before and during administration of the mineralocorticoid.
2. Within the first week of steroid administration an increase in mean arterial blood pressure could be demonstrated, which was due to an increase in cardiac output. After the sixth week the elevated blood pressure was the consequence of an increased total peripheral resistance.
3. Plasma noradrenaline concentration decreased and reactivity to exogenous noradrenaline increased during steroid administration.
4. The mechanism underlying the increase in total peripheral resistance during long-term mineralocorticoid administration remains unclear. Increased sympathetic tone does not seem to be a factor since plasma noradrenaline decreased considerably. Pressor response to noradrenaline increased probably due to decreased sympathetic tone.