It is hypothesized that myocardium subjected to a 5 minute period of coronary occlusion and a 30 minute period of reperfusion has latent abnormalities that become overt when the reperfused myocardium is "challenged" by a subsequent coronary occlusion. This hypothesis is clinically relevant because reperfused myocardium is frequently subjected to recurrent ischemia, as in patients with unstable angina, vasospastic angina or recurrent thrombosis after initial coronary occlusion and thrombolysis. In 19 open chest dogs, the response of regional myocardial function to brief coronary occlusions was studied. Systolic wall thickening and diastolic thinning were measured using a specially developed miniature 5 MHz echocardiographic transducer fixed to the epicardium by suction. All 19 dogs underwent an initial "challenge" coronary occlusion (30 seconds). Thereafter, the control group (n = 8) underwent no intervention for 30 minutes, while the intervention group (n = 11) underwent 5 minutes of coronary occlusion followed by 30 minutes of reperfusion. All dogs were then subjected to a second "challenge" coronary occlusion (30 seconds). In the control group, responses to the second challenge occlusion were the same as to the first occlusion. In the intervention group, regional and global systolic function and myocardial perfusion after the 5 minute coronary occlusion intervention returned to baseline levels, but the response to the second challenge coronary occlusion was significantly different in the intervention group.(ABSTRACT TRUNCATED AT 250 WORDS)