The radiotracer method of Oldendorf was used to determine if 2450 MHz continuous wave (CW) microwave energy increases blood-brain barrier permeability to [14C]mannitol, which is normally excluded from entering the brain. Anesthetized, adult rats were irradiated singly for 30 min in the quiet zone of an anechoic chamber, at average power densities from 0.1 to 30 mW/sq.cm. Afterwards each rat received an intracarotid bolus injection of [14C]mannitol/[3H]water mixture and was decapitated 15 sec later. Uptake of [14C]mannitol relative to the highly permeable reference substance, [3H]water, was calculated as the brain uptake index (BUI) for 4 brain regions. Mean BUI values for tissues from the microwave-irradiated rats did not differ significantly from sham-irradiated animals, and a microwave influence on barrier permeability was not evident. Irrespective of treatment, BUI values for cerebellum and medulla were much higher and more variable than values for cortex or diencephalon, and were associated with reduced absorbance or retention of [3H]water. Because of a compromising influence of the vertebral arterial supply on the distribution of intracarotid-injected radiotracers, BUI measurements in caudal brain regions are probably unreliable unless accompanied by data on regional radioisotope concentrations. The absence of such data in an earlier BUI study, suggests that increases in BUI for cerebellum and medulla attributed to microwaves were possibly misinterpreted as differences in barrier permeability to [14C]saccharides, when in fact changes in blood flow and [3H]water influx or egress were responsible.