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Behavioral effects of 2.45-GHz microwaves (100% amplitude modulated at 120 Hz) were studied at averaged power densities of 8.8, 18.4 and 37.5 mW/cm2 of incident radiation as measured in the absence of the animal subjects. Four rats were exposed for 60 min while performing in a response chamber of Styrofoam. Lever pressing was reinforced by food pellets on a fixed-interval 50-sec schedule and produced high rates of responding in two rats and low rates of responding in the other two rats. Radiation at 37.5 mW/cm2 disrupted the lever response in the two rats that responded at high rates during baseline measures. Radiation at the two lowest power densities had no observable effect on the rate of responding. For the two rats that responded at low rates, no effects on rate of lever pressing were observed at any of the power densities. However, ambulatory activity of all rats decreased during radiation at 18.4 and 37.5 mW/cm2. This decrease was associated with a tendency of the rats to remain in areas of the response chamber with lower power densities of irradiation. The results are believed to be due to an interaction between higher metabolic rates in the more frequently responding rats and exogenous heating by microwave radiation.