The purpose of the present study was to evaluate potential diurnal changes in resting coronary blood flow under conditions of comparable myocardial oxygen requirements. We studied 21 conscious, resting dogs instrumented for the measurement of coronary flow, left ventricular pressure, and regional myocardial segment length. Recordings were taken in the early morning and late afternoon. In the afternoon, left circumflex coronary blood flow was 12.8% higher than in the morning, without detectable changes in the major hemodynamic determinants of myocardial oxygen consumption, i.e., heart rate, left ventricular pressure, peak positive dP/dt, end-diastolic regional myocardial dimension, and regional myocardial stroke work. These results are consistent with a circadian variation of coronary vascular tone accompanied by compensatory alteration in myocardial oxygen extraction and may have clinical relevance, since the coronary vascular tone may change dynamically in patients with critical coronary stenosis.