1. The direct effect of catecholamines and other vasoactive substances on superficial hand veins has been studied in man. Drugs were infused locally into the vein and changes in vein size at a constant congesting-pressure were measured by a simple optical technique. 2. Noradrenaline, adrenaline, 5-hydroxytryptamine and angiotensin II caused venoconstriction. Large doses of angiotensin II resulted in rapid development of tachyphylaxis. 3. Isoprenaline, bradykinin, acetylcholine and histamine had no effect on the relaxed vein but all caused dilatation when infused into veins preconstricted by noradrenaline or 5-hydroxytryptamine. 4. No effect of vasopressin could be demonstrated.