Pial arterial vessels were inspected in hypercapnia (PaCO2 97 +/- 9 (S.E.) mmHg) and in acute hypertension induced by 5 microgram kg-1 noradrenaline i.v. (mean arterial pressure 210 +/- 5 mmHg) by means of a cranial window. The diameter of arterial vessels with a resting diameter of 10 to 150 micrometer was measured either from photographs or by aid of an image splitting eyepiece. Arterioles with a resting diameter less than or equal to 30 micrometer exhibited the highest degree of dilatation both in hypercapnia and acute hypertension. The mean dilatation was higher in hypertension than in hypercapnia in vessels of all sizes, the difference being statistically significant in arterioles up to 50 micrometer resting diameter.