The receptors for the phorbol ester tumor promoters are present in highest concentration in the brain. We have examined the distribution of receptors in mouse brain by light microscopic autoradiography. Marked differences in receptor levels were found both between brain regions and between layers of a single region. Binding was high in hippocampus, cerebral cortex, striatum and substantia nigra, lower in thalamus and pons. In cerebellum, binding was high in the stratum moleculare, intermediate in the stratum granulosum and low in the lamina alba. The localization suggests a functional role for the receptors within brain, and it permits comparison with other known activities. For example, the pattern of localization is distinct from those reported for neurotransmitter receptors or for phospholipids.