The effect of intra-arterially administered prostaglandin F2α(PGF2α) on venous demonstration in angiography of the superior mesenteric artery was compared to that of bradykinin in 10 patients, and the flow increase in this vessel following PGF2α was measured by a dye dilution technique in another 8 patients. The angiographic demonstration of veins was similarly improved by the 2 drugs, the effect of 60 µg. of PGF2α being roughly equivalent to that of 10 µg. of bradykinin. The loss of arterial detail was less marked after PGF2α which, however, had a longer duration of action. The flow studies indicated that 80 µg. of PGF2α increased the superior mesenteric blood flow 103±16 per cent. There was no influence on blood pressure or pulse rate. The patients experienced a sensation of warmth in the abdomen for a few seconds following the injection of the drugs, but no other side effect was noted. It is concluded that PGF2α causes a rapid increase of the superior mesenteric blood flow in man making it suitable for pharmacoangiography of this vascular bed.