Three mature Thoroughbred geldings were given 13.63 mg phenylbutazone/Kg bodyweight intravenously for 3 days and repeated in one horse 4 days later. After 4, 7 and 10 days (double treatment), degeneration of the wall of small veins occurred in all horses. The veins were dilated and/or showed hyalin degeneration. The phlebopathy was interpreted to be paramount in phenylbutazone intoxication. All other manifestations, including erythro- and leukodiapedesis, submucosal edema and ulceration of the gastrointestinal mucosa, phlebothrombosis and significant changes in the hemogram and serum chemistry, were considered secondary to the vein lesions.