Experiments employing guinea pig heart Langendorff preparations compared the coronary vasoactivity of a functionalized congener of adenosine, 2-[(2-aminoethyl-aminocarbonylethyl)phenylethylamino]-5'-N-e thyl- carboxamidoadenosine, APEC, with the vasoactivity of the product of the reaction of APEC with 1,4-phenylene-diisothiocyanate, 4-isothiocyanatophenylaminothiocarbonyl-APEC (DITC-APEC). Previous experiments showed that whereas APEC binds reversibly to the A2A adenosine receptor of brain striatum, DITC-APEC binds irreversibly. APEC caused concentration-dependent coronary vasodilation that persisted unchanged when agonist administration continued for up to 165 min, but promptly faded when the agent was withdrawn. The unselective adenosine receptor antagonist 8-(4-sulfophenyl)theophyline (8-SPT) antagonized the vasoactivity of APEC. By contrast, DITC-APEC (0.125-1.0 nM) caused progressive, concentration-independent vasodilation that persisted unchanged for as long as 120 min after the agent was stopped and that was insensitive to antagonism by subsequently applied 8-SPT. However, perfusion of the heart with buffer containing 0.1 mM 8-SPT strongly antagonized the coronary vasodilatory action of DITC-APEC given subsequently. Such observations indicate that the covalent binding of DITC-APEC causes irreversible activation of the guinea pig coronary artery A2A adenosine receptor. Neither APEC nor DITC-APEC appeared to desensitize the coronary adenosine receptor during two or more hours of exposure to either agonist.