The regional distribution of prostaglandins E and F in 24 different areas of the rat brain was studied. For bioassay the rat uterine horn preparation was used. Prior to bioassay the samples were purified with organic solvents and in some cases by chromatography on silica as well. For measurement of PGF2 alpha content in certain areas a radioimmunoassay was used. Each region was found to contain prostaglandins but in uneven concentrations. Highest concentration, related to the protein content, was present in the median eminence, 57.6 ng PGE and 34.4 ng PGF per mg protein. Parietal cortex, olfactory tubercle, cerebellar cortex, caudate nucleus, substantia nigra, and paraventricular nucleus are also rich in prostaglandins. Low values were found in frontal cortex, thalamus, and septum. The distribution of prostaglandins throughout the areas examined do not show any direct correlation with that of transmitter substances. The uneven distribution can be attributed to differences in the endogenous biosynthetic capacity of each region.