Coronary blood drains through three types of channels: the subepicardial veins, the thebesian veins, and the arteriosinusoidal vessels. In this study the changes in the blood draining through the subepicardial and thebesian veins were measured in relation to time, up to 180 minutes. Analysis of the data yielded useful information about the relatively inaccessible microcirculation in the working heart. The first group of eight dogs was subjected to pulsatile perfusion and the second to nonpulsatile perfusion. The arteriovenous oxygen difference decreased at an identical rate of 0.01 ml of oxygen per minute per 100 ml of blood in both groups. Spontaneous increase in drainage (milliliters of blood per 100 gm of myocardium per minute) occurred as follows: Subepicardial venous drainage increased at a rate of 0.34 +/- 0.03 (mean +/- SE) with pulsatile perfusion and at a rate of 0.23 +/- 0.03 with nonpulsatile perfusion; the values were significantly different (p < 0.025). Corresponding values for thebesian venous drainage were 0.08 +/- 0.01 with pulsatile perfusion and 0.06 +/- 0.01 with nonpulsatile perfusion (p < 0.05). As a result, there was a linear increase in total myocardial oxygen utilization (MVO2) with pulsatile perfusion and a decrease with nonpulsatile perfusion. The increase in drainage with nonpulsatile perfusion, therefore, may have been due predominantly to abnormal shunt activity. The difference in drainage increase may then represent nutritive flow, the absence of which led to a fall in MVO2 with nonpulsatile perfusion. There was also some evidence that the "thebesian system" may play a compensatory role during shunting. Abnormal shunting of blood may be partly responsible for the perfusion-related myocardial damage reported in the literature.