The time course of uptake, retention and clearance of the cationic lipophilic dye, rhodamine 123 (Rh123), within the central nervous system was qualitatively evaluated in rats. Weanling rats were injected intracerebrally with avian sarcoma virus, which induced malignant gliomas in situ before injection of Rh123. Comparison was made of the amount of fluorescence of Rh123 within the normal cerebral cortex, myelinated tracts of the brain, meninges, choroid plexus, and neoplastic foci at 1, 4, 8, 12 and 24 hours after intravenous injection. Fluorescence microscopy was utilized to identify tissues containing the dye. Normal neuropil did not contain Rh123 at any of the time periods studied. Gliomas retained the dye at 1, 4, 8 and 12 hours, with increasing uniformity of distribution and decreasing intensity of fluorescence over this time period. Fluorescence was not detected at 24 hours within the neoplastic tissues, but was evident at all time periods studied within the choroid plexus. The specific retention of Rh123 by malignant glioma and by the choroid plexus in vivo suggests that Rh123 may be useful for photochemotherapeutic treatment of brain neoplasms and disorders of the choroid plexus.