Regional cerebral blood flow was studied in Lewis rats with fulminant acute experimental allergic encephalomyelitis (EAE). [14C]iodoantipyrine was used as a tracer. By employing a short experimental time and an infusion schedule producing an increasing arterial tracer concentration, the spatial resolution of the method was fine enough to detect focal increases in blood flow in the small central nervous system lesions (lymphocytic accumulations). An increase of flow of 100% in the lesions and a decrease of 50% in the cerebral cortex of EAE animals was statistically significant. In all other regions studied (deep cerebral structures, cerebellum), blood flow in EAE animals did not differ from the control values. The flow increase corresponding to the lesions may be due to inflammatory hyperemia. The cortical decrease in flow may be secondary to sensory motor impairment.