The distribution of adenosine A1 receptors in the human brain was studied by autoradiography in post mortem brain tissues from 26 subjects without reported neurological disease. N6-[3H]Cyclohexyl-adenosine was used as the ligand. For comparison, adjacent sections of some regions were examined histochemically for 5'-nucleotidase activity. The receptor sites were heterogeneously distributed throughout the CNS. The highest receptor densities were found in the stratum oriens, pyramidale and radiatum of the hippocampus. High densities were also found in the cerebral cortex and the striatum. In the thalamus there was a heterogeneous distribution of binding sites with a high density in structures such as the medial and anterior nucleus. Intermediate receptor densities were found in the accumbens, the olfactory tubercle and most parts of the amygdala among others. The hypothalamus had low receptor densities. In the brainstem and the spinal cord very low receptor concentrations were found. However, in some structures such as the substantia nigra, the colliculus superior and the substantia gelatinosa of the spinal cord a low level of binding could be measured. The cerebellar cortex showed low densities of receptors. Structures showing high levels of 5'-nucleotidase activity were the hippocampus, the striatum and parts of the cerebral cortex among other regions. In general there was a poor correlation between the localization of A1 receptors and the 5'-nucleotidase activity. Some regions, however, showed a similar distribution of these two markers. In general, the distribution of adenosine A1 receptors found in the human brain is comparable to that found in previous autoradiographic studies in the rat brain. However, some regional differences were observed in, for example, the cerebral cortex, the striatum and the cerebellar cortex. These differences may prove to be functionally relevant.