The structure of 20 second-order arterioles from the hamster cheek pouch was quantitated after histological fixation at physiological pressures. Dilated and contracted vessels with respective internal diameters of 66 ± 9.2 and 30 ± 9.1 μm (SD) were studied. Electron microscopy of four dialted arterioles revealed a combined intima-media thickness of 3.2 ± 1.5 μm with the following composition determined by stereological point counting: endothelial cells (20%), endothelial basement membrane (3%), a continuous elastic lamina (8%), smooth muscle cells (49%), and medial extracellular space (20%). Light microscopy of whole mounts of cheek pouches and isolated arterioles stained with hematoxylin was used to measure orientation and lengths of both smooth muscle and endothelial cell nuclei. The smooth muscle orientation was not significantly different in dilated or contracted arterioles. For all of the muscle orientation measurements combined, the mean pitch angle was 1.1 ± 5.9° (N = 1000) with respect to the circumferential axis of the vessel. Comparison of dilated and contracted arterioles indicated a significant decrease in the muscle cell and nuclear length, but no detectable change in either orientation or nuclear length of the longitudinally oriented endothelial cells. Wall areas measured from transverse sections of dilated and contracted vessels did not change upon contraction. Stereological estimates of relaxed smooth muscle cell length averaged approximately 60 μm. The manner in which structural parameters affect arteriolar mechanical properties is discussed, and it is concluded that, in hamster pouch arterioles, over 99% of the developed smooth muscle force is transmitted circumferentially.