Squirrel monkeys, Saimiri sciureus, were trained to respond on a two-lever observing-response task for food pellets. After stable response rates developed, the monkeys were exposed in the far field to 2.45-GHz microwaves (100% sinusoidally modulated at 120 Hz) at average power densities from 10 to 75 mW cm−2 of incident radiation. Four monkeys were exposed to irradiation for 30 minutes during two-hour sessions. Three of the monkeys were also exposed for 60 minutes during two-hour sessions. The animals were restrained in Styrofoam chairs during experimental sessions and exposures occurred in a microwave-anechoic chamber. The behavior of the monkeys on the observing-response task was disrupted during 30- or 60-minute exposures to irradiation but only at power densities that were 50 mW cm−2 or higher. This disruption was increasingly evident as power density increased. Under both durations of exposure, behavior was not consistently perturbed until rectal temperatures increased more than 1°C. Rectal temperature was slightly but reliably elevated at 10 mW cm−2, was a monotonie function of the power density, and was markedly increased at power densities between 40 and 50 mW cm−2.