The distribution of [3H]alpha,beta-methylene ATP ([3H]alpha,beta-MeATP) binding sites in rat brain and spinal cord was examined using autoradiography. It was shown that many structures in the CNS are densely labelled, and that the binding is displaced by both beta,gamma-methylene ATP and the P2X-purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), indicating that the binding sites are P2X-purinoceptors. Image analysis revealed that the nuclei of the thalamus have the highest density of labelling. Other densely labelled structures include: the amygdaloid, substantia nigra, cerebral cortex, hypothalamus, caudate putamen, geniculate nuclei, medial habenula and the intermediate zone of grey matter in the spinal cord. These results are consistent with the limited amount of electrophysiological evidence for the existence of P2X-purinoceptors in the CNS, and provoke the need for extensive physiological studies to establish the roles of P2X-purinoceptors in the CNS.