The cells of twelve arterioles have been reconstructed from serial Araldite sections of 0.5 µm thickness stained with toluidine blue. Each arteriole had a single layer of smooth muscle cells. Each cell was spindle-shaped, contained one nucleus and was wound around part of the circumference of the arteriole. A 30-µm length of arteriole contained 19.1 ± SEM 2.2 nuclei, and therefore the same number of smooth muscle cells. This number did not vary with ãrteriolar internal diameters in the range 11–31 µm. The maximal axial width of the cells was found by inspection to be 4.2 ± 0.2 µm and this too was independent of the ãrteriolar diameter. From the number and width of the cells and the surface area of the arteriole, the length of the cells was calculated to be 47 ± 6 µm. The length was greater in the large arterioles and was 90% of the circumference on average. The thickness of the smooth muscle cells in the radial direction was 3.3 ± 0.2 µm. The helical pitch of the cellular axes to the axis of the arteriole was 81.3 ± 1.5 °. These results are discussed in relation to the tension developed in ãrteriolar walls and in relation to the electrotonic spread of current from cell to cell along an arteriole that has been measured with intracel