The purpose of the present study was to characterize adenosine receptors in human atrial and ventricular myocardium. In isolated electrically driven preparations, adenosine produced „direct” negative inotropic effects in atrial myocardium (AT). In ventricular myocardium (VE), it only had negative Inotropic properties when force of contraction had been stimulated with isoprenaline (“indirect” effect), but it has no inotropic effect alone. The adenosine receptor antagonist 8-phenyltheophylline antagonized the „direct” and „indirect” effects; these findings indicated that both effects were mediated by adenosine receptors. In cardiac membranes from human AT and VE, adenosine receptors were characterized with [3H]-8-cyclopentyl-l,3-dipropylxanthine (DPCPX) binding. The effects of agonists.R-(-)-N6-phenylisopropyladenosinc (R-PIA),S-(+)-N6-phenylisopropyladenosuie (S-PIA), and 5′-(N-ethylcarboxamido)adenosine (NECA) and the effects of guanine nucleotides [Gpp(NH)p] were studied also. The antagonist affinities as judged from the apparent affinity, Kd, of [3H] DPCPX were similar in AT (2.2 nmol/1; 95% confidence limits, 1.4–3.7) and VE (1.8 nmol/1; 95% confidence limits, 1.0–3.0). The number of adenosine receptors was 1.7 times greater in AT (26.9 ± 2.33 fmol/mg protein;n=5) than in VE (16.2 ± 23 fmol/mg protein;n=5). High and low affinity states of adenosine receptors evaluated with the influence of Gpp(NH)p on agonist competition withR-PIA were similar in AT or VE. The rank orders of potency for agonists (R-PIA>S-PIA>NECA) and antagonists (DPCPX>8-phenyltheophylline>theophylline) were characteristic for the A, receptor subtype. It is concluded that A, adenosine receptors exist in the human myocardium. Since binding properties were similar in AT and VE, the same A1adenosine receptor probably couples to different effectors in a similar guanine nucleotide-dependent way. [3H] DPCPX is the first radiolabeled antagonist ligand that allows detection of A, adenosine receptors and their coupling in the human myocardium.