Intraarterial occlusion-infusion of vincristine and Adriamycin was performed in six dogs using balloon infusion catheters. Tissue toxicity was compared with that obtained when the same drugs were administered by simple arterial infusion. Balloon occlusion of the external iliac artery produced reversed blood flow in the deep femoral, caudal femoral, and superficial circumflex iliac arteries, potentially altering the distribution of the drugs. Thrombosis of arterial branches occurred in three dogs, sooner but no more often in the groups receiving occlusion-infusion chemotherapy. Skin ulceration and dilatation of peripheral small vessels were significantly more frequent and extensive in the groups receiving occlusion-infusion. One infusion dog developed a single small ulcer on the hind paw; five of the six occlusion-infusion dogs developed multiple skin ulcers on the thigh and/or distal extremity. These findings suggest that even though intraarterial occlusion-infusion chemotherapy is not applicable in all instances, the technique does produce significantly greater drug-related effects than simple intraarterial infusion.