We have studied contractile responses to norepinephrine (NE) and K⁺ of isolated rabbit ear arteries and veins in an effort to determine how α -adrenoceptor affinity and smooth muscle contractility affect responsiveness at different temperatures. Arteries have predominantly α₁-adrenoceptors and veins have predominantly receptors of the α ₂-type. We obtained dose-response curves to NE at 9, 16, 23, 30, 37 and 42 °C in the absence and presence of the irreversible α -adrenoceptor blocker phenoxybenzamine (POB). In both vessels, affinity of the α -adrenoceptors was determined by comparing equieffective doses of NE before and after blockade of the receptors. In other experiments, we obtained contractions to 80 mMKCl at the same temperatures. Affinity of the ear arterial α₁-receptors was maximal at 23 °C, while affinity of ear venous α₂-receptors increased linearly with cooling to 9 °C. Responses to K⁺ in both vessels decreased with cooling. Taken together, the changes in receptor affinity and response to K⁺ may explain the shape of curves relating contractile strength to temperature.