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 less than 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 less than 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.