Isolated dog hearts (n = 19) were preserved in cold Collins solution at 4 degrees C for 1 to 26 hours. At the end of preservation the coronary system was flushed out by cold cardioplegic solution, and the hearts were reperfused with supporting dogs. Myocardial biopsies at the end of preservation and coronary sinus effluents at the time of flush-out were analyzed for adenosine 5'-triphosphate and inosine. Both the degree of depletion in adenosine triphosphate content and the accumulation of inosine in the myocardium during preservation correlated significantly to the preservation time (correlation coefficient = -0.85 and 0.86, respectively). There was a significant correlation between the myocardial inosine level and total amount of inosine released into coronary sinus effluent (correlation coefficient = 0.91, p less than 0.001). Also, a significant correlation between the total amount of inosine in coronary sinus effluent and cellular level of adenosine triphosphate was found (correlation coefficient = -0.86, p less than 0.001). Five hearts that failed to resume beating showed significantly higher amounts of released inosine than the other hearts (3.3 +/- 0.9 versus 1.5 +/- 0.5 mumol, p less than 0.005). These results indicate that the inosine level in the coronary sinus effluent reflects the myocardial energy state, and it is suggested that the measurement of inosine level in the effluent may be applicable as a noninvasive method for assessment of myocardial viability in heart preservation.