OBJECTIVE The effect of pentoxifylline (PTX) on the blood flow in experimental rodent tumors was investigated. METHODS When the R3230 AC adenocarcinoma implanted in the leg of Fischer 344 rats grew to about 1 g, the effect of PTX on the blood flow in the tumor and in the skin and muscle was determined with the microsphere method using 85Sr labelled 25 microns diameter microspheres. The SCK mammary carcinoma was induced subcutaneously in the leg or foot of A/J mice and the effect of PTX on the tumors was investigated: the blood perfusion in the leg tumors (7 mm in diameter) was determined with the 86Rb uptake method and that in the foot tumors (5 mm diameter) was determined with the laser Doppler flow (LDF) method. RESULTS The blood flow in the R3230 AC adenocarcinoma significantly increased when measured 30 min after an IP injection of 50 mg/kg PTX while the blood flow in the normal skin and muscle remained unchanged. The 86Rb uptake in the SCK tumor slightly increased 30 min after an IP injection of 50 mg/kg PTX. The LDF in the SCK tumors grown in the foot began to increase 5-10 min after an injection of 25 mg/kg PTX reaching 1.5-2.0 times in 20-30 min and it returned to the original level at 60 min. CONCLUSIONS The results in the present study together with our previous observation that PTX increases the tumor pO2 in rodent tumors strongly suggest that PTX may be useful for increasing the radiosensitivity of human tumors.