The effects of St 155, guanethidine and reserpine on the reflex vasodilatation and constriction elicited by various vasoactive substances have been studied in auto-perfused hindlimbs of cats anaesthetised with pentobarbitone. After acute administration of guanethidine and reserpine, both the constrictor and dilator reflexes were abolished. In contrast, St 155 at 10 μg/kg caused an initial depression of the vasodilator reflex lasting about 30 min, followed by a prolonged augmentation. Higher doses produced only blockade, the duration of which increased with dose. The drug-induced vasoconstrictor reflex, on the other hand, was always augmented. The reflex pressor response caused by carotid occlusion was markedly depressed by all three hypotensive agents. In animals that had been chronically treated with guanethidine or reserpine, reflex hindlimb responses were never observed. However, both reflex vasodilatation and constriction were still apparent after chronic treatment with St 155. The reflex constriction could be augmented by a further dose of St 155 given during the experiment. St 155 given acutely produced a marked hypotension, while reserpine and guanethidine had little effect. Given chronically, however, all three drugs caused a large fall in systemic blood pressure. Reserpine at 2 mg/kg, St 155 at 10 μg/kg and guanethidine at 0.5 mg/kg had no effect on the hindlimb vascular resistance, although higher doses of the latter two drugs caused a depression. It is concluded that the primary hypotensive action of St 155 does not depend on a reduction of sympathetic tone to peripheral resistance vessels. It is suggested that the ability of St 155 to leave vasoconstrictor reflex intact may explain the lack of postural hypotension following the use of this drug.