The effects of intraarterial chemotherapy with cisplatin on rabbit VX2 carcinoma transplanted to the hind legs of animals were studied. Following intraarterial (IA) or intravenous (IV) administration of cisplatin (2.5 mg/kg), changes in the tumor cells were examined electron microscopically. The treated cells showed enlargement with decrease in the amount of nuclear chromatin and prominent nucleolar segregation. In many cells, the cytoplasmic organelles had migrated to the paranuclear area. The most striking change was the appearance of numerous microvilli-like cytoplasmic projections. Multinucleated giant cells were also observed. The changes were more prominent following IA cisplatin. The author concludes that significant ultrastructural changes can be demonstrated by the regional administration of cisplatin in rabbit VX2 carcinoma and that these promising results should be followed by clinical trials.