blood pressure was 220/170 mm Hg, he had grade IV hypertensive retinopathy, and the serum creatinine level was 5 0 mg/100 ml. He still felt well. Diazoxide 300 mg intravenously lowered the blood pressure, but methyldopa, debrisoquin, reserpine, and clonidine failed to maintain control. There was pronounced orthostatic hypotension, but the blood pressure in the lying posture was high. Renal biopsy showed severe hypertensive nephropathy with many hyalinized glomeruli, the serum creatinine rose to 11-3 mg/100 ml, and there were bouts of left ventricular failure. It was decided to perform bilateral nephrectomy, and an infusion of sodium nitroprusside readily controlled the blood pressure (Table II). At the time of writing he was awaiting a renal transplant and his blood pressure was under control without drugs.Case 4.-A 48-year-old man had a 10-month history of hypertension and renal failure. Control of hypertension had become increasingly difficult, and on admission the blood pressure was 230/ 150 mm Hg, the serum creatinine 13-0 mg/100 ml, and there was a grade IV hypertensive retinopathy. Pentolinium, 25 mg subcutaneously in 12 hours, had no effect on the blood pressure, but it fell briefly to 90/70 mm Hg after 300 mg of diazoxide intravenously. Sodium nitroprusside was then used with good effect. It was continued during peritoneal dialysis preparatory to bilateral nephrectomy, but there were pronounced swings in blood pressuresystolic 80-260, mean 156 (S.D.43); diastolic 50-160, mean 112 (S.D.28)-with severe anginal pain. There was no evidence of myocardial infarction. Sodium nitroprusside was continued after dialysis ended and blood pressure control again became satisfactory and the anginal pain ceased. After bilateral nephrectomy his blood pressure returned to normal without further therapy, but unfortunately he died six days after the operation from bronchopneumonia and septicaemia.Discussion Sodium nitroprusside has a direct depressant effect on vascular musculature, independent of the nervous system (Page et al., 1955). The effect is immediate and ends when the infusion is stopped. The latter property is of special value in patients in whom the cause of hypertension is to be abruptly removed. I have not used the drug during surgery for a phaeochromocytoma, but it would seem to be the drug of choice for this purpose and for when controlled lowering of blood pressure is needed during surgery. Though it is less effective in normotensive subjects than in hypertensives it produces a fall of blood pressure, and Kaneko et al. (1967) used it rapidly to lower the blood pressure in normotensive subjects to study renin release.The drug is virtually non-toxic during short-term use. Long-term toxic effects are due to accumulation of thiocyanate, to which sodium nitroprusside is converted. One case reported by Nourok et al. (1964) developed high serum levels of thiocyanate and features of hypothyroidism after 21 days, which improved when the nitroprusside infusion was stopped. The longest period of infusion in my cases ...