Pentoxifylline, a methylxanthine derivative, was given intravenously to cats, starting 24 hours before or immediately after cerebral edema was induced in one hemisphere by a standard freezing lesion. Pentoxifylline caused a reduction of the cerebral edema, with preferential involvement of the gray matter. Treatment given during 72 or 96 hours after the lesion was made reduced the increase in water and sodium content and also reduced the decrease in potassium content in the injured hemisphere. It is suggested that the reduction of the edematous changes caused by pentoxifylline is mainly due to the inhibition of 3',5'-AMP-phosphodiesterase, to the enhanced cerebral blood flow, and to the inhibition of alpha-adrenergic effects.