The transient decrease in left circumflex coronary artery blood flow during atrial contraction (atrial coves) was examined in open-chest, heart-blocked dogs. Prominent atrial coves were observed in left circumflex flow during both diastole and systole as a result of the asynchrony of atrial and ventricular contractions. In eight open-chest dogs, atrial contractions decreased diastolic circumflex coronary flow by 0.016 +/- 0.002 ml and systolic flow by 0.014 +/- 0.001 ml compared with diastolic and systolic intervals during which no atrial contractions occurred. The diastolic and systolic flow reductions in this vessel were not significantly different (P greater than 0.14). In five of the eight dogs no diastolic atrial coves were observed in left anterior descending coronary flow, and only minimal diastolic coving was present in the remaining three dogs; no systolic coves were present in this vessel in any of the eight dogs. In five additional open-chest dogs, brief inflation of a balloon in the left atrium produced elevations of left ventricular diastolic pressure comparable to those produced by atrial contraction. Only minimal atrial coves were associated with these balloon inflations. These data are consistent with the hypothesis that atrial contraction limits flow through the atrial blood vessels and results in a transient decrease in flow in the atrial arteries that arise from the left circumflex coronary artery. Although increases in ventricular diastolic pressure may be partially responsible for the appearance of atrial coves in the left circumflex coronary artery, mechanical contraction of the atrial musculature appears to be of significant importance.