The participation of coronary perfusion pressure in hemodynamic and transmural metabolic changes was examined in the open chest canine heart. At lower pressure than 50% of the control, reactive hyperemia disappeared and the coronary arterial inflow precipitously decreased. In 8 dogs, when the coronary arterial inflow was decreased to 47-49% of the control by coronary constriction for 15 min, the coronary perfusion pressure fell in various degrees ranging from 39 to 73% of the control. At a lower pressure than 60% of the control, creatine phosphate (CP) content and its ratio of subendocardium (ENDO) to subepicardium (EPI) decreased, while lactate (LA) content and its ratio of ENDO to EPI increased depending on the degree of the fall in coronary perfusion pressure. A little decrease in ATP content was produced only in the subendocardium under the lower pressure than 50% of the control. When the CP content decreased to below one-third of the control, the significant accumulation of lactate and the precipitous decrease in ATP occurred. Our results suggest that the coronary perfusion pressure has an important role for a severity of the transmural energy metabolism in the hypoperfused ventricle.