The objectives of this study of triphenyltetrazolium chloride (TTC) gross histochemical staining of myocardial infarcts were to determine the accuracy of sizing of established infarcts, to determine the time course of development of staining defects during infarct evolution, and to determine the effect of clinically relevant autolysis on the detectability of early infarcts. Transverse sections of excised rat hearts after left coronary artery occlusion were incubated in TTC for 5 minutes at 37 C. After 48 hours, planimetrically determined infarct size (percent left ventricle [LV%]) by TTC and by conventional histologic study (H) correlated well: TTC (LV%) = 1.04 X H(LV%) - 2.94 (r = 0.98; P less than 0.001). Ischemic zone staining defects, detected in 50% of hearts 30 minutes after occlusion, were noted in all hearts occluded 3 hours or longer. Gross staining contrast was not diminished by in situ autolysis for 6 hours at ambient temperature or for 24 or 72 hours at 4 C. In conclusion, TTC reliably permits quantitation of the size of completed infarcts. TTC often detects ischemic injury as early as 30 minutes after coronary occlusion in the rat and in all hearts by 3-6 hours. Infarct demarcation by TTC is preserved during simulated clinical autolysis.