Bifunctional intramyocardial potassium ion (K+)-sensitive and bipolar wire electrodes were used to evaluate extracellular K+ dynamics and elcctrophysiologic changes during acute myocardial ischemia in the border zone, ischemic zone (5 to 7 mm from the border), central ischemic zone (15 to 25 mm from the border) and normal myocardium in 11 open chest dogs during a 30 min ligation of the left anterior descending coronary artery. At the end of this period, the hearts were injected with rhodamine dye and quickly frozen. Ultraviolet NADH (nicotinamide adenine dinucleotide) rhodamine fluorescence photography was used to localize the border between normally perfused and ischemic tissue and determine the site of electrodes in relation to this border. Before coronary ligation, extracellular K+ ranged from 4.0 ± 0.3 to 4.3 ± 0.3 mM in these four zones. After ligation, extracellular K+ accumulated in the ischemic and central ischemic zones in a pattern characterized by an initial rapid increase for approximately 5 min, followed by a slowly rising plateau phase, reaching maximal levels of 9.8 ± 2.0 and 14.4 ± 4.4 mM, respectively. In contrast, K+ dynamics in the border zone showed a biphasic response, with an initial rapid increase to a maximal level of 7.5 ± 2.4 mM at approximately 9 min after coronary ligation, followed by a gradual decrease to a level of 5.3 ±1.2 mM by the end of the 30 min ligation period. No significant changes in K+ occurred in the normal zone throughout the ischemic period. The correlation of K+ electrode, electrophysiologic and postmortem NADH-rhodamine fluorescence data indicated the existence of a well defined border zone. Thus, K+ dynamics exhibit pronounced heterogeneity among the ischemic zone, border zone and neighboring normal zone and have characteristic temporal patterns that facilitate delineation of a border zone.