Myocardial infarctions were produced in dogs by occluding the left anterior descending artery; the dogs were killed at times varying from 30 min-8 days. Prior to sacrifice Thioflavin S was injected i.v. The excised heart was scanned by a B-scanner interfaced with a computer that quantified signal amplitude. The heart was sectioned, photographed and studied pathologically. Infarcted myocardium showed high ultrasonic reflectivity with average amplitudes 1.4-2.6 times that of normal muscle. Perfusion-histomorphologic evidence of infarction correlated best in infarcts of 24 h or less; older infarcts were underestimated. Tissue changes, sources of false-positive and false-negative findings, signal processing and potential clinical utility are discussed.