The characteristics and distribution of adenosine A1-receptors in human brain tissues were examined, using [3H]N6-cyclohexyladenosine ([3H]CHA) as a ligand. The binding of [3H]CHA had the pharmacological characteristics of an A1-receptor site and was similar to those of adenosine A1-receptors in rat brain tissue examined in parallel. Autoradiographic localization of adenosine A1-receptors in human brain tissues revealed a heterogeneous anatomical distribution with high levels, particularly in the hippocampal formation, striatum, neocortex and some thalamic nuclei. The distribution of receptors was similar to that seen in the rat brain. However, in some regions, as for example the cerebellar cortex, clear differences were seen.