The right or left coronary arteries were constricted by cholinergic agents and dilated by adrenergic agents. The receptors mediating cholinergic responses are muscarinic. Comparison with the ear artery indicates that the adrenergic receptors mediating dilatation in the coronary artery resemble β₁ rather than β₂. However, the ratios of the dilator potencies of noradrenaline, adrenaline and isoprenaline on the coronary artery were 1:0.23:2.6 compared with ratios of their positive inotropic potencies of 1:1.1:33 on the rabbit papillary muscle. The markedly smaller relative potency of noradrenaline on the papillary muscle may be due to uptake₁, since the potentiating effect of cocaine on noradrenaline on this tissue was approximately 12-fold, but was negligible on the coronary artery. The contribution of uptake₂ is unclear, since histochemical observations indicated selective accumulation of noradrenaline in the wall of the artery, whereas the dilator responses were not appreciably altered by uptake₂ inhibitors.