A method is described to measure regional blood flow in different structures of the rat brain. Microspheres (15 micron) are injected, the brain is sectioned, stained for myeline, radioautographs are prepared and the microspheres in the different structures are counted. The values obtained for different brain structures are counted. The values obtained for different brain regions (cortex, corpus callosum, thalamus hipocampus, hypothalamic region, colliculi, cerebellum, pons, medulla) compare well with those published by others on larger animals. In rats fed 1% of lead from birth, higher blood flow is found in the cortex and a lower one in the interior part of the brain compared to controls.