Adenosine, potassium ions and hydrogen ions are known to be vasoactive in the coronary circulation. Little is known, however, about the combined effects of these chemicals during reactive hyperemia, flow autoregulation or hypoxic hyperemia. Isolated, perfused guinea pig hearts were used to study the influence of simultaneously administered theophylline, ouabain and alkalosis on coronary flow responses (occlusive hyperemia, autoregulation, hypoxic hyperemia) thought to be contributed to by the above chemicals. Retrograde aortic inflow was monitored electromagnetically in the absence and presence of concurrently administered theophylline (5 × 10<sup>–5</sup> M), ouabain (1.4 X 10<sup>–7</sup> M), and alkalosis (perfusate pH 7.69). Upon release of a 30-sec inflow occlusion (in the presence of the above agents) mean peak coronary flow was modestly, but significantly, reduced. Volume flow rate and time for flow to return 50% towards control were reduced in the presence of the above agents, but failed to return to pre-experimental values upon removal of blockers. When perfusion pressure was increased from 65 to 95 cm H<sub>2</sub>O (antagonists present), coronary myogenic autoregulation was significantly enhanced. Upon decreasing pressure from 95 to 35 cm H<sub>2</sub>O, in the presence of blockers, calculated coronary resistance fell from 13 ± 0.6 to 9.5 ± 0.5 cm H<sub>2</sub>O/ml/min. The latter value was significantly higher than that seen in the absence of test agents. No difference in the maximum mean coronary flow achieved by hypoxia (20% O<sub>2</sub>–5% Co<sub>2</sub>-balance N<sub>2</sub>) was seen in the absence or presence of blockers. In conclusion, these experiments provide some indirect support for the involvement of the above metabolites in reactive hyperemia and as antagonists of myogenic blood flow autoregulation. Little evidence for their involvement in hypoxic hyperemia is available from this study.