The aims of the present study were to quantitate blood flow rate and capillary permeability of <sup>99m</sup>Tc-DTPA in the human myocardium and to assess whether capillary permeability is influenced by the presence of small degree atherosclerotic lesions. Myocardial blood flow rate and capillary permeability of <sup>99m</sup>Tc-DTPA were quantitated during coronary angiography by the single injection, residue detection method. Eighteen patients undergoing a diagnostic coronary angiography because of uncharacteristic chest pain were studied. In 12 patients with angiographically normal epicardial coronary arteries, a mean value of the capillary permeability-surface area (PS) product of 54.0 ml·100 g-min)<sup>-1</sup> (SD 13.0) was determined from a capillary extraction of 55.0% (SD 9.4%) and a regional myocardial plasma flow rate of 74.6 ml·(100 g·min)<sup>-1</sup> (SD 6.3) equivalent with a regional myocardial blood flow rate of 121.7 ml·(100g·min)<sup>-1</sup> (SD 13.0). Similar values of blood flow rate, capillary extraction fraction and the PS product were determined in 6 patients with localized coronary atheroma without hemodynamically significant coronary artery stenosis (25-50% luminal narrowing). The values for the regional myocardial blood flow rate in the human heart are in accordance with values obtained by other methods, and the PS product of <sup>99m</sup>Tc-DTPA is similar to results obtained in canine hearts. Localized coronary atheroma leading to insignificant coronary artery stenosis does not produce any measurable dysfunction of the coronary microcirculation, in terms of capillary permeability and blood flow rate, during resting conditions.