The present study assesses effects of acutely infarcted myocardium on apparent microsphere loss as a function of time, determines effects of apparent microsphere loss on blood flow measurements to ischemic regions, and determines to what extent apparent microsphere loss alters interpretation of serial measurements of collateral blood flow. Studies were performed in 35 awake mongrel dogs chronically instrumented with catheters in the aorta and left atrium and an occluder on the proximal circumflex coronary artery. Myocardial blood flow was measured before and 15 minutes after complete occlusion. Dogs were randomly divided into four groups to be killed at 6 and 24 hours, and 3 and 6 days. In the 3-day group, an additional blood flow measurement was made 24 hours postocclusion. The entire left ventricle was sectioned into 1- to 2-g samples and myocardial blood flow determined. The ratio of preocclusion blood flow in each ischemic sample to mean nonischemic flow was used to calculate apparent microsphere loss and to correct ischemic blood flow in each sample. Significant apparent microsphere loss occurred in epicardial layers at 24 hours and in epi- and endocardial layers at 3 and 6 days; maximum loss at each interval was 22.3, 19.4, and 22.2% respectively. Absolute blood flow corrections for ischemic myocardium were small, range -0.035 to 0.083 ml/min per g. Changes in flow to ischemic regions between 15 minutes and 24 hours were comparable before and after correction for apparent microsphere loss. Although infarction resulted in significant apparent microsphere loss, effects on ischemic blood flow measurements were very small and consequently did not prevent interpretation of serial blood flow measurements after infarction in animals killed at 3 days.