The present study was designed to determine if estramustine phosphate (EMP) could potentiate the effects of irradiation on the Dunning (R3327) prostatic adenocarcinoma in rats. Two groups of male Copenhagen x Fisher F1 rats carrying bilateral tumors in the flank were used. Irradiation was given with a linear accelerator 6 MV, in a dose of 6 Gy/day for 4 days to the tumor on one side while the tumor on the other side served as control. EMP (360 micrograms/24 hours) was administered with osmotic pumps to one group of rats for 2 weeks, starting 1 week before irradiation. Tumor growth was calculated by measuring tumor volume, and tumor blood flow was measured 8 weeks after treatment. Irradiation alone effectively delayed tumor growth and EMP enhanced these effects. Tumor blood flow was stimulated by EMP treatment irrespective of radiotherapy. Volume density of tumor epithelium was effectively decreased by irradiation but no significant effects could be seen after EMP. In conclusion, the present study shows that EMP potentiates irradiation on rat prostatic adenocarcinoma, and further evaluation of this therapeutic approach in the clinical treatment of prostatic carcinoma is thus justified.