Intravenously injected [14C]sucrose was used as a small molecular weight (342 daltons), hydrophilic tracer for determination of 2450 MHz CW microwave and ambient heat effects on rat blood-brain barrier permeability in the cerebral cortex, hypothalamus, cerebellum and medulla. The tracer was injected 4 min following exposure of conscious, unrestrained rats to microwaves at 0 or 65 mW/cm2 for 30 or 90 min (SAR approximately equal to 13.0 W/kg) or to ambient heat (42 +/- 2 degrees C) for 90 min. Comparison of mean permeability-surface area products (PA) and uptake ratios between sham and microwave-exposed animals revealed a statistically significant (P less than 0.05) decrease of both PA and uptake ratios for the hypothalamus, cerebellum and medulla of rats exposed to microwaves for 30 min. This decrease was not apparent for rats exposed to microwaves for 90 min. A pertinent observation, with regard to this latter group of animals, was the increased circulating levels of the tracer when colonic temperature was raised to approximately 41.4 degrees C or higher.