AbstractA comparison of the effects of continuous‐wave, sinusoidal‐amplitude‐modulated, and pulsed square‐wave‐modulated 591‐MHz microwave exposures on brain energy metabolism was made in male Sprague‐Dawley rats (175–225 g). Brain NADH fluorescence, adenosine triphosphate (ATP) concentration, and creatine phosphate (CP) concentration were determined as a function of modulation frequency. Brain temperatures of animals were maintained between −0.1 and −0.4°C from the preexposure temperature when subjected to as much as 20 mW/cm2(average power) CW, pulsed, or sinusoidal‐amplitude modulated 591‐MHz radiation for 5 min. Sinusoidal‐amplitude‐modulated exposures at 16–24 Hz showed a trend toward preferential modulation frequency response in inducing an increase in brain NADH fluorescence. The pulse‐modulated and sinusoidal‐amplitudemodulated (16 Hz) microwaves were not significantly different from CW exposures in inducing increased brain NADH fluorescence and decreased ATP and CP concentrations. When the pulse‐modulation frequency was decreased from 500 to 250 pulses per second the average incident power density threshold for inducing an increase in brain NADH fluorescence increased by a factor of 4—ie, from about 0.45 to about 1.85 mW/cm2. Since brain temperature did not increase, the microwave‐induced increase in brain NADH and decrease in ATP and CP concentrations was not due to hyperthermia. This suggests a direct interaction mechanism and is consistent with the hypothesis of microwave inhibition of mitochondrial electron tr