The effect of hypoxia and blood flow on the capillary permeability-surface area product (PS) of 51Cr-EDTA was investigated in canine myocardium of open chest anesthetized dogs at constant aortic pressure, heart rate, and cardiac output. PS was determined by bolus injection of 51Cr-EDTA into the left anterior descending coronary artery (LAD) and external registration of the response curve. Vascular conductance (G) and PS were measured: (1) during pump perfusion of LAD with arterial blood (control state), and (2) during vasodilation obtained by LAD perfusion with deoxygenated blood at same blood flow as in control state, and (3) during increased blood flow with deoxygenated blood. Mean value of G in control state was 1.31 ml.min-1 (100 g)-1.(mmHg)-1. The ratio G-hypoxia/G-control used to assess the extent of vasodilation was 2.42 (range 1.67-3.56) during hypoxia and unchanged flow and 2.82 (range 1.81-4.47) during hypoxia and increased flow. Mean value of PS in control state was 36 ml-(100g)-1.min-1. With maximum vasodilation and constant blood flow PS increased to 47.3 ml.(100 g)-1.min-1 (37%) and during increased blood flow to 69.0 ml.(100 g)-1.min-1 (96%). The increase in PS most likely reflects an increase in capillary surface area available for exchange of 51Cr-EDTA indicating a 1.4- to 2-fold recruitment of capillaries.