Single smooth muscle cells from bovine coronary artery were obtained by enzymatic digestion. 20.1 ± 3.2% (mean value of seven experiments) of the dispersed cells were in a relaxed, noncontracted state. These cells responded to electrically induced depolarization with a graded contraction to a maximum shortening of 47.8% of initial length; the corresponding maximum for K<sup>+</sup>-induced depolarization was 61.7% of initial length. Cells relaxed only a small percentage (≈ 20%) of the amount of shortening, but could be restimulated several times to contract to even shorter lengths. The decrease in mean cell length of dispersed cells exposed to serotonin, acetylcholine, and ergonovine maleate was 21.0, 23.0 and 25.5%, respectively (p < 0.05). No significant decrease in cell length was obtained with norepinephrine. The ultrastructure of the relaxed cells seemed to be unaltered, suggesting that the accuracy of the physiological results is not impaired by induced structural damage.