OBJECTIVE: The effectiveness of chemotherapeutic agents is impaired by limited delivery of chemotherapeutic agents to the tumor cells. Improving drug penetration in tumor tissues is very important. We tested whether losartan, a selective antagonist against type 1 angiotensin II receptors (AT1R) with noted antifibrotic activity, can enhance the penetration and efficacy of doxorubicin. MATERIALS AND METHODS: BALB/C mice, which implanted with CT26 tumor cells, were divided into four groups: control, doxorubicin alone, losartan alone and doxorubicin + losartan combination groups. At day 0, the losartan alone and doxorubicin + losartan combination groups received losartan; and at day 8, the doxorubicin alone and doxorubicin + losartan combination groups received doxorubicin i.v. Tumor growth and intratumoral distribution of doxorubicin were evaluated. The mechanism underlying the enhanced anti-tumor effect of the combination of doxorubicin and losartan was investigated by immunohistochemical analysis. RESULTS: Treatment with losartan alone did not suppress tumor growth; In contrast, treatment with doxorubicin alone decreased tumor growth; losartan and doxorubicin were administered in combination, had a synergistic effect that the tumor growth was much more inhibited. The decreased proliferation as indicated by down-regulation of Ki67, and increased apoptosis as indicated by TUNEL and caspase-3 staining. The expression of tumor suppressor gene P53 increased in doxorubicin + losartan combination groups. CONCLUSIONS: Losartan can increase the therapeutic effectiveness of doxorubicin, yielding more great antitumor benefit. This study provided a rationale for initiating clinical trials using losartan in combination with chemotherapeutic agents to increase their therapeutic effectiveness.