We performed the present study to characterize phasic venous outflow patterns in conscious chronically instrumented dogs. An implantable ultrasonic transit-time flow probe capable of measuring volumetric venous outflow was placed on the coronary sinus (n = 9) and/or great cardiac vein (n = 5). Under resting conditions, great cardiac vein flow was predominantly systolic [systolic index 61 +/- 6%/beat (means +/- SD), diastolic index 39 +/- 6%/beat]. In contrast, coronary sinus volumetric flow was predominantly diastolic (systolic index 36 +/- 13%/beat, diastolic index 64 +/- 13%/beat). During adenosine vasodilation both great cardiac vein and coronary sinus flow patterns were predominantly systolic (systolic index 58 +/- 12%/beat for the coronary sinus and systolic index 75 +/- 6%/beat in great cardiac vein flow). In addition, the time at which peak venous flow occurred shifted from end systole to midsystole after vasodilation. The variation in coronary venous flow patterns with position suggests that outflow patterns are significantly influenced by a venous compliance between the great cardiac vein and distal coronary sinus. The lack of a predominantly systolic outflow pattern in the coronary sinus under resting conditions contrasts with previous findings in open-chest anesthetized animals and raises the possibility that cannulation of the coronary sinus and/or changes in vasomotor tone caused by anesthesia alter resting venous flow patterns.