Flow reserve of the right coronary artery (RCA) was studied in open-chest dogs and compared with that of the left anterior descending coronary artery (LAD). The maximum percent peak reactive hyperemia (％PRH) of the RCA was greater, and the duration of occlusion that produced one-half of the maximum coronary vasodilation (T1/2) in the RCA was about twice that in the LAD. With an increase in right ventricular MVO2 produced by construction of pulmonary artery or isoproterenol infusion, T1/2 of the RCA was shortened significantly. For estimating the quantitative contribution of the perfusion pressure to coronary flow reserve, we occluded the RCA for 60 sec and the LAD for 30 sec, during which each vessel reached maximum vasodilation with various perfusion pressure in the range of 100 to 20 mmHg. ％PRH decreased with a reduction in perfusion pressure, and perfusion pressure below which the RH abolished was significantly lower in the RCA than the LAD : 32.2±5.7 VS 41.5±5.0 mmHg. Thus, the RCA has a greater flow reserve than the LAD, and lower cardiac work and less extravascular artery compression of the right ventricle would account for this finding.