Several lines of evidence indicate that the extent of ischemic injury is not defined immediately after arterial occlusion, but that infarction expands over time. Episodes of spreading depression have been linked to this secondary increase in infarct volume. Tissue bordering the infarction fails to repolarize following spreading depression and is incorporated into the lesion. The result is that ischemic infarctions expand stepwise after each episode of spreading depression. Another line of evidence has demonstrated that gap junction blockers effectively inhibit spreading depression. These observations suggest that traffic of potentially harmful cytosolic messengers between ischemic cells and surrounding nonischemic cells might cause amplification of injury in focal stroke. It is therefore conceivable that minimizing gap junction permeability might reduce final infarct volume. To test this hypothesis, the authors pretreated rats with the gap junction blocker, octanol, before occluding the middle cerebral artery and compared the sizes of the ischemic lesions to those in rats that received the vehicle, dimethyl sulfoxide, prior to arterial occlusion. Histopathological analysis was performed 24 hours later. The 12 octanol-treated animals showed a significantly decreased mean infarction volume (80 +/- 16 mm3) compared with the nine control rats (148 +/- 9 mm3). In a separate set of experiments, the frequency of experimentally induced waves of spreading depression was evaluated after octanol treatment. Octanol pretreatment resulted in complete inhibition in two of nine animals, transient inhibition in five, and no inhibition in two. The results indicate that gap junction inhibitors, when not limited by toxicity, have significant therapeutic potential in the treatment of acute stroke.