Two rhesus monkeys were irradiated in a 383 MHz resonant cavity immediately before and during performance of a four-choice, forced-choice serial reaction task. Integral dose rates of .001 through 17.5 W were delivered to the head. No effects were observed below a critical dose level (about 23 W/kg) derived from integral dose rate and body mass. Above this level, behavioral suppression occurred, i.e., correct response rate was profoundly altered. The effect was completely reversible and repeatable in one of the subjects - the other subject did not recover completely and was sacrificed for histological examination which revealed no gross or microscopic damage. The nature of the effect suggests a neurochemical rather than an electrical or mechanical basis for the results.