The effects of intraperitoneal injection of physiologic saline solution, pentobarbital, 50 mg/kg, or thiopental, 20 or 50 mg/kg, upon small arteries (35–45 μ) and snail veins (70–100 μ) in the wings of unanesthetized bats were studied. The rate of venous vasomotion and the diameters of the artery and vein were recorded for 20 minutes before, and for 50 minutes after, each injection. Dilatation of the artery to 129.3 ± 6 per cent of control (P< 0.02) and the vein to 124.1 ± 7.3 per cent of control (P<0.02) occurred 10 minutes following pentobarbital. Thiopental (50 mg/kg) increased venous vasomotion (108 ± 1.3 per cent,P< 0.02), but neither dosage of thiopental produced a significant change in the diameters of the artery and vein. It is concluded that pentobarbital and thiopental have different effects upon the small blood vessels of the subcutaneous tissues in the bat wing.