In previous experimental work, the Ca-antagonist Nimodipine had shown a predominantly cerebroarterial dilatory effect. In the present double-blind study of 16 patients, pial arterial and venous reaction was investigated during EC-IC bypass surgery, infusing 1 microgram kg-1min-1 of Nimodipine intravenously. In pial arteries with resting diameters between 25 and 70 micrometers, a significant 18% dilatation was observed. Results are considered promising for future trials in the treatment and prevention of cerebral ischemia caused by vasoconstriction or vasospasm, especially vasospasm following subarachnoid hemorrhage.