Conditions such as Raynaud's disease, cold intolerance following digit replantation, and reflex sympathetic dystrophy are all exacerbated by diminished blood flow in the digits. The rabbit ear, being similar to the human digit, is an excellent model for studying microvascular responses during thermoregulation and was used in this study of microcirculatory changes during total-body cold stress. In eight rabbits a window was placed in one ear each, and the underlying vascular beds were photographed with the unanesthetized animal at room temperature (23 degrees C) and in a cold room (6 degrees C). Luminal arterial diameters constricted more than venular diameters during cold stress (39.6% vs. 16.5%, respectively, of the control diameters; P = 0.014). All responses were linear; the slopes of the regression lines were significantly different between arterioles and venules (P < 0.0001). This study suggests that decreased circulation in response to cold is due to decreases in the diameter and number of vessels available for perfusion, as predicted by the Poiseuille equation.