We studied the effects of combination chemotherapy of an antitumor drug cis-diamminedichloroplatinum (II) (DDP) and its potent antidote, sodium thiosulfate (STS) in rat liver tumor systems. This therapy was given to female WKA rats with metastatic liver tumors 13 days after inoculation of syngeneic hepatoma cells through the mesenteric vein. DDP and STS were administered via two different routes, hepatic artery and femoral vein, respectively (we call this treatment "two route infusion chemotherapy"). The antitumor effects were evaluated 21 days after the treatment by calculating the tumor weight from the total weight of the liver. Tumor weights of rats treated with 20 mg/kg of intra-arterial DDP plus 1,054 mg/kg of systemic STS (group A), 5 mg/kg of intra-arterial DDP alone (group B), and 5 mg/kg of systemic DDP alone (group C) were, about one fifth, two fifths and three fifths of the tumor weights in the untreated controls, respectively. In group A, no rats died despite administration of a 4-fold higher DDP dose than in the latter two groups B and C in which 14-18 per cent of the rats died, due to DDP-induced toxicity. The patterns of body weight gain in the three groups after the chemotherapy were much the same. Our results clearly indicate that the antitumor effect of DDP on metastatic liver tumors in rats can remarkably be enhanced by the "two route infusion chemotherapy" of DDP and STS.