The effect of a varying period of ischemia on the development of myocardial infarction was investigated in the rabbit. Radiomicrosphere measurements confirmed that the collateral blood flow is almost zero (0.02 +/- 0.01 ml/min/g) and without a significant transmural gradient in the rabbit heart (n = 15). A coronary branch of the left circumflex artery was occluded for 5, 10, 15, 30 or 60 min and then reperfused. The coronary branch was occluded permanently in another group of rabbits. Three days after the coronary occlusion, the infarct size was determined by hematoxylin-eosin and Mallory's staining and the ischemic zone size was determined by fluorescent particles. The results showed that the percentage of the ischemic zone infarcted (% infarction) vs the log of duration of ischemia yielded a sigmoid curve which could be linearized by probit analysis: Probits of % infarction = 3.05 x log (ischemic duration in minutes) + 0.33, r = 0.83, p less than 0.01. The regression equation indicated that 50% of the ischemic myocytes necrotized after 34 min of coronary artery occlusion. Unlike in the dog heart, the infarct of the rabbit heart first appears in the midmyocardium and then progresses towards both the endocardium and epicardium.