The amplitude and distribution of epicardial ST-segment elevation (STP) were examined for an 8-hour period after coronary occlusion in eight baboons and five pigs. STt was determined from unipolar epicardial electrograms obtained from a high-resolution matrix of fixed electrodes overlying a transmural region of ischemia. A relatively uniform degree of STt was observed overlying the ischemic region for 20 minutes after coronary occlusion. A gradient in STt from the periphery to the center of the ischemic region was documented after 20 minutes of ischemia. In 10 other pigs, change in the degree of STt was examined contingent on either an increase (five pigs) or decrease (five pigs) in the size of the ischemic region after 1 hour of preexisting ischemia. An abrupt increase in the number of electrodes that showed STt (NST) from 7.8 i 1.24 (SEM) to 14.8 ± 1.35 (90%) was associated with an increase in mean STt of 58% from 4.28 ± 0.61 mV to 6.78 ± 0.84 (p < 0.05). An abrupt decrease in NST from 25.2 ± 2.63 to 14.6 ± 2.22 (42%) was associated with a decrease in mean STt of 24%, from 8.2 ± 0.36 mV to 6.3 ± 0.30 mV (p < 0.01). The results during early ischemia (less than 20 minutes of ischemia) are accurately represented by a model of ischemia in which injury current arises only at the ischemic boundary. The results during later ischemia (after 20 minutes of ischemia) may be represented by a model in which STR is considered dependent on injury currents generated throughout the ischemic region.