2-[3H]Chloroadenosine was used as a ligand to investigate the presence of adenosine receptors on rat brain cortical membranes. The binding studies reveal the presence of a single binding site which has an apparent dissociation constant (KD) of 23.5 nM and a maximal binding capacity (Bmax) of 476 fmol/mg protein. This binding can be inhibited by adenosine, adenine, nicotinamide adenine dinucleotide phosphate, inosine, theophylline, and isobutylmethylxanthine. These findings strongly support the suggestion that there is an adenosine receptor on brain cell membranes.