Presence and localization of the prostacyclin receptor in the rat brain and spinal cord were examined by in vitro autoradiography using [3H]iloprost, a highly specific and stable agonist for this receptor. Density of specific binding sites for iloprost was generally high in four regions of the lower brain stem, that is, the medial and commissural subnuclei of the nucleus tractus solitarius, the area postrema, superficial layers of the spinal trigeminal nucleus caudalis and dorsal horn. Moderate density was found in the thalamus, cerebral cortex, hippocampus, striatum and dorsal cochlear nucleus. The distribution pattern was distinct from those of other prostanoid binding sites previously studied except that prostaglandin E2 binding sites were also abundant in the nucleus tractus solitarius, spinal trigeminal nucleus caudalis and dorsal horn. Even in these regions, binding sites for iloprost had several features clearly different from those for prostaglandin E2. First, within the medial and commissural subnuclei of the nucleus tractus solitarius, iloprost binding sites were distributed preferentially in the dorsal part, while those for prostaglandin E2 were located more ventrolaterally. Second, on postnatal day 0, iloprost binding sites have already been expressed in large amounts in the nucleus tractus solitarius, spinal trigeminal nucleus caudalis and dorsal horn of rats, while prostaglandin E2 binding sites are negligible at this stage. Thirdly, the binding of 10 nM [3H]iloprost in these three regions was almost completely displaced by 10 microM unlabelled iloprost but only slightly by 10 microM unlabelled prostaglandin E2. Unilateral nodose ganglionectomy or dorsal rhizotomy decreased the density of iloprost binding sites in the nucleus tractus solitarius or dorsal horn, respectively, with a greater decrease in the operated side. Ligation of the vagus either central or peripheral to the nodose ganglion resulted in an accumulation of iloprost binding sites proximal to the ligation. These results suggest that specific binding sites for iloprost, presumably prostacyclin receptor, are present in the nervous system and, in particular, that the iloprost binding sites in the nucleus tractus solitarius, dorsal horn and possibly in the superficial layers of the spinal trigeminal nucleus caudalis are produced in their sensory ganglia and transported to central terminals of the primary sensory afferents as well as to their peripheral terminals.