The mechanism of renin release was studied in 8 hypertensive patients following the administration of diazoxide. Plasma renin activity, urinary sodium excretion (Una V), plasma electrolytes, plasma volume, the change in extracellular fluid (ECF) volume and blood pressure were measured. Renin release was found to be independent of any measured change in plasma or ECF volume. The signal to release renin in this situation appears related to the fall in blood pressure, the fall in plasma Na concentration or to the fall in UNaV. Three patients failed to increase renin following diazoxide administration.