In the present study it was investigated whether hypertension induced by dopamine or hypotension induced by clonidine could influence the relative blood distribution in a s.c. transplanted prostatic adenocarcinoma in relation to blood flow in normal tissues in rats. The blood flow was measured by using a radioactive microspheres technique. A bolus injection of 5 micrograms clonidine caused a decrease in the systemic blood pressure with 15-45 mm Hg, and blood flow to the central parts of the tumor, decreased with more than 50%. In the prostate, testes and ileo-psoas muscle a similar reaction was seen. During an infusion of 95-120 micrograms/kg/min dopamine, the blood pressure increased with 15-30 mm Hg, but the blood flow to the tumor was unchanged. However, in the testes and kidneys a decreased blood flow was observed. Moreover, mean blood flow to the central parts of the tumor decreased with increase in tumor size. The results further indicated that hyper- or hypotension or concomitantly given vasoactive drugs with the specific cancer treatment can have unwarranted effects on the distribution of cytotoxic drugs and oxygenation of the tumor.