Biphasic vasodilatory responses to adenosine and 5′-N-ethylcarboxamidoadenosine (NECA) were observed in the coronary vasculature of K+-arrested perfused rat hearts. Dose-response data for both agonists were best represented by two-site models. For adenosine, two sites with negative log ED50 (pED50) values of 8.1 ± 0.1 (mean ± S.E.M) and 5.2 ± 0.1 were obtained, mediating 31 ± 2% and 69 ± 2% of the total response. In the presence of 8-phenyltheophylline, the vasodilatory response to adenosine remained best fitted to a two-site model with pED50 values of 7.0 ± 0.2 and 5.4 ± 0.2. The relative contribution of each site to the total response remained unchanged. For NECA, pED50 values of 9.6 +- 0.1 and 6.8 ± 0.2 were obtained, representing 48 ± 3% and 52 ± 3% of the sites, respectively. In contrast, ATP produced a monophasic response with a pED50 value of 8.8 ± 0.1. These results provide evidence of adenosine receptor and response heterogeneity in the in situ coronary vasculature.