To determine the extent to which thermoregulatory vasoconstriction decreases heat loss to the environment, we measured regional heat flux, average skin temperature, and tympanic membrane temperature before and after thermoregulatory vasoconstriction in five minimally clothed volunteers maintained in a 30.8 ± 0.1° C environment. Thermoregulatory vasoconstriction was induced by central venous infusion of cooled fluid. Peripheral cutaneous blood flow was evaluated with venous-occlusion volume plethysmography and skin-surface temperature gradients. Laser Doppler flowmetry was used to measure vasoconstriction in centrally located skin. This model mimics the common clinical situation in which patients in a warm environment are centrally cooled by administration of cold intravenous fluids or by lavage of internal cavities with cold fluids. Tympanic membrane temperature decreased 1.5 ± 0.3° C in the first 15 min after the cold fluid infusion was started and remained ≈1° C below control values during the rest of the study. Average skin-surface temperature decreased slowly to ≈0.7° C below control. Flow in capillaries of centrally distributed skin, determined with laser Doppler flowmetry, decreased only ≈40%. Total heat flux, and flux from the arms and legs decreased ≈25% (15.5 ± 0.3 W). Heat loss from the trunk and head decreased only 17%, whereas, loss from the hands and feet (10.5% of the body surface area) decreased ≈50%. All measured values decreased significantly following vasoconstriction (P < 0.01). Therefore, thermoregulatory vasoconstriction in a thermoneutral environment appears to decrease cutaneous loss of metabolic heat ≈25%.