The vasomotor response of the tail of the albino rat to total-body heating and cooling was studied by skin-temperature recording and plethysmography with the tail at 25 °C air temperature. Tail vasodilatation started at core temperatures slightly above 37 °C and increased to a core temperature up to about 39 °C. During cooling of warm rats, tail vasoconstriction started at significantly higher levels of core temperature than the values at which vasodilatation appeared when the rat was warmed.