In isolated guinea pig hearts saline perfused at constant flow, adenosine A₁, A_2A, and A₃ (A_x) agonists covalently bound to a large polymer (Pol; 2,000 kDa) were intracoronarily administered, and three effects were studied: dromotropic, vascular and inotropic. The rank order of potencies were the following: dromotropic (Pol-A_2APol-A₁>Pol-A₃) and vascular and inotropic (Pol-A_2APol-A₁Pol-A₃), where the rank order of potency for Pol-A_x depends on the part of the coronary vascular network involved; i.e., there is a vascular heterogeneity. The large size of Pol-A_x prevents extravascular diffusion and causes it to act solely in the endothelial luminal surface. This implies their cardiac effects are due to endothelial mediators. Inhibition of nitric oxide (NO) and prostaglandin (PG) synthesis with N^G-nitro-L-arginine methyl ester and indomethacin, respectively, show that the three cardiac effects of Pol-A₁ were mediated by NO and PG, whereas for Pol-A_2A and Pol-A₃ the mediator was mainly NO but not PG. These results suggest that if Pol-A_x activated the corresponding endothelial A_x-receptor subtype, a different mediator would be produced.