In rabbits and guinea pigs, hypothalamic sites for prostaglandin Ej (PGE2) action were studied by means of in vitro receptor autoradiography. The density of PGE2 binding sites (probably PGE2 receptors) was the highest in the anterior wall of the third ventricle (A3V). This result is consistent in all mammalian species ever studied, suggesting a fundamental role of the A3V in the hypothalamic action of PGE2, such as fever.