A series of experiments was conducted in rats to observe their responses to 2450-MHz (CW) microwaves. Colonic temperatures were measured after exposure to 0, 20, or 30 mW/cm2 for up to 4 h. Exposures of 30 mW/cm2 caused a regular and significant increase in colonic temperature. A plateau of 38.9C was reached in 90 min and maintained for the rest of the 4-h exposure. Rats exposed to 20 mW/cm2 for 4 h exhibited patterns of colonic temperature similar to sham-exposed rats. Acclimation for 90 min had no effect on these patterns. Measurements (or calculations) of 26 serum chemistry values produced similar results in rats exposed to 30 mW/cm2 for 120 min and rats exposed to 0 or 20 mW/cm2 for 120 min. Only serum corticosterone was significantly increased in the dose-related response of naive rats to this acute exposure. Various injectable anesthetics were evaluated as restraints in microwave exposure experiments. Colonic temperature was a measure of the response to exposure. Immobilization was accompanied by decreased colonic temperature. After exposure to 30 mW/cm2 for 90 min, mean colonic temperatures in chemically restrained rats ranged from 39-40.4C, and increased up to 5.5C from anesthetically-depressed pre-exposure levels. Limitations on the use of chemical restraint for this kind of experiment and on the use of multivalue serum chemistry screening tests were determined in the course of this research.