AbstractSquirrel monkeys (Saimiri sciureus) trained to regulate environmental temperature (Ta) behaviorally were exposed in the far field of a horn antenna to ten‐minute periods of 2,450 MHzCW microwaves. Incident power density ranged from 1 to 22 mW/cm2. The corresponding specific absorption rate (SAR), derived from temperature increments in saline‐filled styrofoam models, ranged from 0.15 to 3.25 W/kg. Controls included exposure to infrared radiation of equivalent incident energy and no radiation exposure. Normal thermoregulatory behavior produces tight control over environmental and body temperatures; most monkeys select a Taof 34–36°C. Ten‐minute exposures to 2,450 MHzCW microwaves at an incident power density of 6–8 mW/cm2stimulated all animals to select a lower Ta. This threshold energy represents a whole‐body SAR of 1.1 W/kg, about 20% of the resting metabolic rate of the monkey. Thermoregulatory behavior was highly efficient, and skin and rectal temperatures remained stable, even at 22 mW/cm2where the preferred Tawas lowered by as much as 4°C. No comparable reduction in selected Tabelow control levels occurred during exposure to infrared radiation of equal incident