Contemporary reports in the scientific and popular press of potentially hazardous effects of exposure to microwaves require substantiation because some Navy personnel contact a variety of microwave devices in communication, warning and weapons systems. Such putative effects preclude the use of man as a subject; hence, a series of experiments with other primates, monkeys, has been initiated. Research in our laboratory has established that microwave irradiation greater than 62 mW/sq cm disrupts behavior in rhesus monkeys. In an effort to extend the generality of this finding, squirrel monkeys are exposed to microwaves. The behavior of squirrel monkeys on a vigilance task was disrupted by 30- or 60-minute exposures to 50 mW/sq cm and higher power densities. This disruption increased with the increase in power density. Under both durations of exposure, behavior was not consistently perturbed until colonic temperature changes exceeded 1 C. Colonic temperatures regularly increased beginning at 10 mW/sq cm and were related in a nonlinear fashion to the power density with a marked acceleration between 40 and 50 mW/sq cm.