Twenty-three patients with evolving acute myocardial infarction (AMI) undergoing catheterization for thrombolytic therapy had interventional contrast ventriculography using programmed atrial stimulation. Postextrasystolic (PES) potentiation was present in 67% of infarct-related segments up to 9 hours after the onset of AMI. The presence of segmental potentiation was not related to time from onset of pain to ventriculography, initial ejection fraction, presence of collaterals, left ventricular end-diastolic pressure or the PES delay. In 18 patients reperfusion was successful using intracoronary streptokinase an average of 6.2 hours after the onset of AMI; in these patients repeat contrast ventriculography was performed an average of 11 days after AMI. Improved chronic segmental ventricular function was predicted by the presence of collaterals to the infarct-related artery at the time of acute catheterization (p = 0.02), but was best predicted by analysis of acute PES potentiation (p less than 0.0001). The predictive value of PES analysis was highest in segments without collaterals. Thus, atrial stimulation is safe during AMI and analysis of segmental ventricular function shows potentially viable myocardium up to 9 hours after the onset of AMI. In addition, analysis of PES segmental function can predict chronic function if reperfusion is successful, especially in segments without collaterals. PES ventriculographic analysis may allow prospective determination of which patients during AMI are most likely to benefit from acute thrombolytic therapy.