Electrotherapy with low-level direct current (DC) can induce antitumor effects in various tumor models. Applied in combination with certain anticancer drugs, it can significantly increase their effectiveness. It has been suggested that the demonstrated effects of electrotherapy arise from its modification of tumor blood flow. The effect of such treatment on blood perfusion of solid subcutaneous Sa-1 fibrosarcoma tumors in A/J mice was investigated with a &rubidium extraction technique. Following electrotherapy, the relative tissue perfusion of tumors was decreased by more than 50%. Three days after treatment, partial reperfusion of tumors occurred. The dynamics of the perfusion changes induced by electrotherapy are in agreement with tumor growth dynamics following this procedure. The effect of electrotherapy on the blood supply of tumors may be the major mechanism of antitumor action in our model. Electrotherapy could be useful as an adjuvant local procedure to other treatment modalities that require a hypoxic environment for their effectiveness.Address correseondence to: Toma; Jarm, University of Ljubljana, Faculty of Electrical Engineering, Triaska 25, SI-lo00 Ljubljana, Slovenia. Fax: +386 (0)61 1264-658.