The distribution of serotonin receptors in the rat brain was studied using a new, quantitative autoradiographic technique utilizing tritium-sensitive LKB sheet film, and densitometric analysis of the film images. OD values above 13 were found in the dentate gyrus, subiculum, substantia nigra and ependymal tissue throughout the brain. ODs between 7 and 11 were found in the globus pallidus, anterior hippocampus and CA1 field, dorsal raphe nucleus and interpeduncular nucleus. The lowest densities were observed in the thalamus, cerebellum, and over white matter, between 0.1 and 0.4 OD units. Other brain nuclei have intermediate values. Serotonin receptor distribution is found to correlate with 5-HT levels, corticosteroid and estrogen receptors in some but not all brain regions.