The importance of protein extravasation for the development of vasogenic brain edema is still controversial. We, therefore, assessed the cerebrovascular permeability to serum proteins in relation to the development and resolution of brain edema in a photochemical cortical lesion in the rat. Cortical infarction was induced by in situ thrombosis using an argon laser beam aimed at the exposed parietal bone in animals given rose bengal i.v. The histology and the cerebrovascular permeability to serum proteins were scrutinized from 2 h to 3 weeks after the insult. The presence of serum proteins was demonstrated by an immunoperoxidase technique. The cerebral water content was estimated by specific gravity measurements of the cortical tissue in a kerosene-monobromobenzene gradient column from 2 h to 7 days after infarction. The blood-brain barrier was permeable to proteins at 2 h following the insult and proteins spread into the medial and lateral tissue reaching a maximum at 24 h. The specific gravity did not deviate from control values at 2 h. After 8 h the specific gravity of the lesion decreased with smaller decreases in the immediately adjacent tissue. At 24 h the changes in specific gravities reached a maximum in all regions except the immediately lateral area. The edema was generally worse in tissue medial to rather than lateral to the infarct. The degradation of serum proteins and the resolution of the brain edema followed the same time course with partial resolution of 72 h. By 1 week serum proteins and edema were confined to the central necrotic core. The results suggest a relationship between cerebrovascular permeability and cerebral edema in photochemical cortical infarction.