The relation between myocardial adenosine content, coronary sinus plasma concentration, and coronary vascular resistance was studied in conscious dogs during rest or treadmill exercise. Exercise at 5 mi/h for 7 min on a 20% slope had a significant threefold increase in cardiac work over that of dogs at rest. Exercise increased myocardial adenosine content from 1.35 +/- 0.54 to 8.18 +/- 0.60 nmol/g and decreased coronary vascular resistance from 1.07 +/- 0.13 to 0.69 +/- 0.08 mmHg.ml-1.min.100 g; the former correlated negatively with the latter (r = -0.74, P less than 0.01). The adenosine concentration in coronary sinus plasma increased from 108 +/- 16 to 184 +/- 18 nmol/l plasma and correlated negatively with coronary vascular resistance (r = -0.83, P less than 0.01). Myocardial adenosine content correlated positively with coronary sinus plasma adenosine concentration (r = 0.77, P less than 0.05). Low myocardial lactate content, continued lactate uptake, lack of change in the lactate-pyruvate ratio, and stable coronary sinus oxygen tension values indicated no myocardial hypoxia. These findings demonstrate the release of adenosine into coronary sinus blood and show significant correlations between myocardial adenosine content, coronary sinus adenosine concentration, and coronary vascular resistance during physiological exercise in the dog. The results support the hypothesis that adenosine regulates coronary blood flow during increased metabolic activity.