Significantly elevated levels of sodium fluorescein (MW 376) were found only in the brains of conscious rats made considerably hyperthermic (colonic temperatures greater than 41.0 degrees C) by exposure to ambient heat (42 +/- 2 degrees C) for 90 min or 2450 MHz CW microwave energy at 65 mW/cm2 (SAR approximately equal to 13.0 W/kg) for 30 or 90 min. For microwave-exposed rats, fluorescein levels within the cortex and hypothalamus appeared to increase with increasing duration of exposure. This trend was not apparent in the cerebellum or medulla. Exposure to ambient heat resulted in increased fluorescein with the cortex, hypothalamus and medulla, but not the cerebellum, and, in general, ambient heat was not as effective as microwave energy in raising tracer concentrations within the brain. By far the greatest elevation of fluorescein dye in the brain occurred in those animals whose blood-brain barrier had been opened osmotically by intracarotid injection of 10 M urea. It is suggested that increased levels of sodium fluorescein found in the brain tissue of ambient heat and microwave-exposed rats most likely represent technically derived artifact and not a breakdown of the blood-brain barrier.