The acute peripheral vascular and diuretic effects of intravenous frusemide 10 mg and 20 mg were compared with those of bumetanide 250 micrograms and 500 micrograms in a group of 10 salt depleted volunteers. Significant reductions in forearm blood flow (FBF) were observed after frusemide 10 mg (-0.77 ml 100 ml-1 min-1 P less than 0.05) and 20 mg (-0.75 ml 100 ml-1 min-1 P less than 0.01 at 15 min). No changes were observed after bumetanide. The reductions in blood flow produced by frusemide were significantly different from those of bumetanide (P less than 0.05) at 15 min. Increases in venous capacitance (VC) and mean arterial blood pressure (MAP) were observed after frusemide but these differences were not statistically different from placebo or bumetanide. No increases were seen after bumetanide. Plasma aldosterone concentrations were unchanged after either drug but plasma renin activity (PRA) was increased after frusemide 10 mg (4.42 +/- 1.01----8.50 +/- 1.90 ng A I ml-1 h-1 P less than 0.01) and 20 mg (4.01 +/- 0.72----7.81 +/- 2.27 ng A I ml-1 h-1 P less than 0.05). No increases were observed after bumetanide and significant differences between bumetanide and frusemide were observed (P less than 0.01). This study demonstrates that the acute peripheral arterial effects of frusemide are not observed after comparable diuretic doses of bumetanide. The differences appear to be related to the ability of the drugs to stimulate acute renin release from the kidney.