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The heads of 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. CW radiation at integral dose rates of 0.001 -17.5 W were delivered to the head. No effects were observed below a critical dose level (≈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.