The aim of this study was to modify the photochemical stroke model of Watson et al. [23] so as to make possible microscopical investigation of the so-called penumbra, a tissue zone at risk that surrounds an infarction. The idea was to minimize photochemical challenge to endothelial membranes in such a way that thrombotic vascular obstruction is avoided but destabilization of the blood-brain barrier is still obtained. Morphological examination of the challenged area revealed open blood vessels, overt blood-brain barrier leakage over the entire area, severely swollen glial cells and structurally intact neurons. The lesion expanded over time due to progressive extravasation, formation of perivascular edema and consequent development of secondary ischemia through mechanical compression and microvascular congestion. In contrast to a photothrombotic infarct, in which the ischemic insult is more severe and blood vessels are completely congested by aggregated platelets, with this approach blood flow is partially preserved. In this way, an ischemic penumbra is created that mimics pathologic conditions secondary to stroke and trauma. The model may be useful in studying effects of drugs on pathologic phenomena that are characteristic of a penumbra, e.g. vasogenic and cellular edema, inflammation and infarction.