The reactivity of isolated porcine coronary venous segments to acetylcholine, noradrenaline, and histamine was tested in organ bath experiments. Acetylcholine dilated precontracted venous preparations endothelium-dependently. This relaxation was inhibited by atropine indicating the muscarinic nature of this response. Noradrenaline contracted the vessels by an alpha-adrenergic mechanism, whereas beta-receptor-induced dilatation could not be seen. Histamine induced contraction of porcine venous preparations via H1-receptors. These results suggest that the coronary venous system is involved in the regulation of the coronary blood flow in addition to metabolic mechanisms and to nervous influences on the arterial side.