Myocardial extraction of Tl has been postulated to depend on the rate of delivery (flow rate) and the metabolic state of tissue (ATPase activity). Experiments were performed to assess the role of these factors. In 22 arrested dog hearts the left anterior descending and circumflex coronary arteries were cannulated and pump perfused with oxygenated blood containing 204Tl. Isotope activity was determined in coronary sinus blood. The myocardial extraction ratio (E) of Tl varied inversely with flow, and the permeability-surface area product (PS) increased with increasing flow rates. These findings indicate that Tl uptake is flow dependent and can be analyzed with Renkin's capillary clearance theory. To assess the role of ATPase in Tl uptake, studies were also performed with blood containing ouabain. After introducing ouabain, coronary sinus blood Tl activity increased, approaching arterial activity, and E fell markedly. It was concluded that myocardial Tl uptake is mediated by ATPase.