The effect of intravenously administered papaverine hydrochloride on the regional cerebral blood flow (rCBF) was determined in the contralateral hemispheres of 6 patients with acute unilateral cerebral infarction. In all 6 patients, the resting state rCBF was reduced in the noninfarcted hemisphere. Papaverine increased the hemispheric mean rCBF in only 1 patient, whereas abnormal responses were observed in 5 subjects. Mean hemispheric flow values decreased paradoxically in 2 patients and remained unchanged in 3. Individual rCBF values increased in the entire contralateral hemisphere in 1 patient. Widespread paradoxical rCBF decreases were observed in another subject. Mixed regional responses to papaverine, showing decreases, no change and increases in rCBF values occurred in the opposite hemispheres of 4 patients. These findings provide additional evidence that the chem-ical control of rCBF may be impaired in the contralateral ‘unaffected’ hemisphere soon after the onset of acute cerebral infarction.