1. The acute effect of diazoxide (150–300 mg, intravenously) on renal venous renin has been evaluated in forty-four patients with suspected renal hypertension. 2. In twenty-seven studies which showed no lateralization, diazoxide raised the mean peripheral renin to 175% of control ( P < 0·05) but the renal vein renin ratio showed an insignificant change (1·15 to 1·21). 3. In seventeen studies which showed lateralization the increase in peripheral renin was similar, but mean renal vein renin ratio increased from 1·90 to 3·52 ( P < 0·005), and the mean ratio of contralateral renal vein to peripheral renin showed no change (1·05 to 1·07), indicating persistence of contralateral suppression. 4. Diazoxide accentuates the distinction between lateralizing and non-lateralizing renal vein renin studies.