Attempts to selectively reduce tumour blood flow have, in the past, concentrated on the use of hydralazine. However, although this vasodilator can be highly effective in experimental animals, it is only at such high concentration as to result in a severe and clinically unacceptable reduction in systemic blood pressure. At clinically acceptable levels, the drug appears to produce a small increase in tumour blood flow. We have used the techniques of magnetic resonance spectroscopy as indicators of metabolism and blood flow in a search for vasoactive drugs that would produce an effective reduction in tumour blood flow without causing severe hypotension or other serious side effects. Single injections of either prazosin or CGRP are shown to be substantially more effective than hydralazine in causing a reduction in tumour blood flow without massive reduction in blood pressure. Even more effective was CGRP given by continuous infusion. In this case a three-fold reduction in tumour blood flow could be obtained with a reduction of only 15-20% in systemic blood pressure. All these studies, however, have been made with transplanted animal tumours. Using high-dose hydralazine and primary tumours that were either radiation or chemically induced, we obtained a success rate of only about a 35% in causing selective reduction in blood flow. In contrast, in a transplanted tumour line derived from one of the non-responding radiation-induced primary lesions, the success rate was about 95%, consistent with the majority of animal studies using transplanted tumours.(ABSTRACT TRUNCATED AT 250 WORDS)