Abstract Distribution of the messenger RNA for the prostaglandin E receptor subtype EP 3 was investigated by in situ hybridization in the nervous system of the mouse. The hybridization signals for EP 3 were widely distributed in the brain and sensory ganglia and specifically localized to neurons. In the dorsal root and trigeminal ganglia, about half of the neurons were labeled intensely. In the brain, intensely labeled neurons were found in Ammon's horn, the preoptic nuclei, lateral hypothalamic area, dorsomedial hypothalamic nucleus, lateral mammillary nucleus, entopeduncular nucleus, substantia nigra pars compacta, locus coeruleus and raphe nuclei. Moderately labeled neurons were seen in the mitral cell layer of the main olfactory bulb, layer V of the entorhinal and parasubicular cortices, layers V and VI of the cerebral neocortex, nuclei of the diagonal band, magnocellular preoptic nucleus, globus pallidus and lateral parabrachial nucleus. In the thalamus, moderately labeled neurons were distributed in the anterior, ventromedial, laterodorsal, paraventricular and central medial nuclei. Based on these distributions, we suggest that EP 3 not only mediates prostaglandin E 2 signals evoked by blood-borne cytokines in the areas poor in the blood-brain barrier, but also responds to those formed intrinsically within the brain to modulate various neuronal activities. Possible EP 3 actions are discussed in relation to the reported neuronal activities of prostaglandin E 2 in the brain.