The arterioles and small arteries of the submucosa of the small intestine and the small arteries (vasa recta) of the mesentery have been measured histologically in cases of hypertension and in normotensive controls. Two methods of mensuration have been used: one employing direct measurement of internal calibre and thickness of media of vessels injected and fixed in a state of dilation at systolic pressure; the other employing measurement of the internal elastic lamina by curvimeter and the cross-sectional area of the media by planimeter in uninjected vessels, from which, by calculation, the internal calibre and medial thickness/lumen ratio of the vessel in an idealised state of dilatation to a perfect circle with uncrenated internal elastic lamina can be derived. These measurements show that the medial thickening observed in the hypertensive vessels is due to a true hypertrophy of the media rather than to a state of contracture. When cases of renal and of essential hypertension are compared the pattern of vascular change is seen to differ in the two conditions: while some degree of hypertrophy is present in both throughout the range of vessel sizes examined, it is more marked in renal than in essential hypertension in vessels with an internal radius of less than 100 micron; but in vessels of more than 100 micron radius the position is reversed. This difference in the reaction of the vessels presumably reflects a basic difference in the mechanisms of production of the two types of hypertension.