The purpose of this investigation was to measure the radius and wall thickness of small blood vessels in mesentery and striated (cremaster) muscle in the anesthestized rat. Using the method of image-splitting, the vessel images were sheared at magnifications of 3,OOOx and 6,500x on the video screen. In the resting state, the mean lumen values for the thinnest portion of endothelial capillary were 4.1 ± 1.2µ (SD) in cremaster and 5.6 ± 1.3µ (SD) in mesentery. Lumen and total diameter were also greater at several levels of precapillary arterioles in mesentery than similar vessels in cremaster, suggesting that smaller vessel size in cremaster might be characteristic of this tissue. Vasoconstriction of up to 50% from resting state is associated with an increase in wall thickness and a decrease in lumen to wall ratio due to a disproportional decrease in radius (inner greater than outer radius). Similarly, an increase in vessel radius in a proportion 4 to 1 (inner greater than outer) was sustained up to about 60% of vasodilatation. In 13 out of 14 vessels, little or no change in wall cross-sectional area occurred in the face of marked changes in lumen cross-sectional area (–95%,+205%). This strongly suggests that other modifications such as changes in length, swelling, or shrinkage would be relatively unimportant during dynamic changes in wall thickness.