An investigation was conducted to determine the effects of relatively low power density microwave exposures on various serum components of the Dutch rabbit. Both continous wave and pulsed mode exposures at 2.45 GHzwere used at power densities of 25,10 and 5 mW/cm2. Studies of 10 serum components were performed. Additional studies were conducted on changes in sleeping times of pentobarbital-sedated rabbits at various power densities. Gross and histopathological examinations were performed on representative samples of animals. Changes in the blood chemistry of irradiated animals were consistent with a dose-dependent response to a non-specific thermal stress at all power densities used. Observed physiological response, as well as rectal temperature measurements, indicated that the thermoregulatory capability of the rabbits was sufficient to compensate for the thermal burden at 5 and 10 mW/cm2, but could be overridden by a 2 h exposure at 25 mW/cm2. Pathology findings included a mild, repairable nephrosis in animals exposed at a power density of 25 mW/cm2. A further investigation of analeptic effects at power densities varying from 5 mW/cm2 to 50 mW/cm2 resulted in a statistically significant decrease in sleeping times, apparently proportional to power density below 15 mW/cm2.