Experimental brain infarcts were produced in 12 adult baboons (Papio cynocephalus) by transorbital permanent clipping of the left middle cerebral artery. One group (seven monkeys) received daily injections of 1 mg/kg dexamethasone, starting 1 h after vascular occlusion and continuing till the end of the experiment. Another group (five monkeys) was not treated. One week after vascular occlusion the volume of infarcts and peri-infarct edema was estimated morphometrically on histological sections, using Masson's trichrome stain and the peroxidase-anti-peroxidase (PAP) technique for visualization of serum protein extravasation. In the untreated animals the average volume of infarct was 6.57±4.23% (mean±SD) and the volume of edema 7.83±2.93% of ipsilateral hemisphere. In the treated animals the infarct volume was not different (7.95±3.00%), but the volume of peri-infarct edema was significantly lower (2.82±3.06%,p<0.05). The results obtained indicate that dexamethasone treatment reduces the development of peri-infarct edema but does not influence the size of infarcts.