Vasomotor activity in cutaneous tail veins was indexed by changes in local skin temperature during exposure of the whole body to 12.3-centimeter continuous microwaves. At an ambient temperature (26°C) just below that at which tail vessels normally vasodilate, criterion dilation was initiated by 5-minute exposures to a microwave power density of 8 milliwatts per square centimeter. This intensity deposits energy equivalent to approximately 20 percent of the monkey's resting metabolic rate but produces no observable change in deep body temperature. Intensity increments of 3 to 4 milliwatts per square centimeter for 1°C reductions in ambient temperature below 26°C produced identical responses. That no vasodilation occurred during infrared exposures of equivalent power density suggests that noncutaneous thermosensitive structures may mediate microwave activation of thermoregulatory responses in the peripheral vasomotor system.