Contractile responses of dog basilar artery in forms of ring and spiral segments to electrical stimulation and vasoactive substances were studied. Ring segments always produced a larger contraction compared with spiral segments in response to various stimuli, while spiral segments produced contraction whose degree depended upon the angle of cut. The dose-response curves for electrical and pharmacological stimulations were almost identical for both ring and spiral segments when the contractile responses were expressed in relative terms, e.g. percent maximum and median effective dose (ED50). Functional and histological studies indicate that the difference in tension developed between ring and spiral segments is primarily attributable to the orientation of the smooth muscle cells. The results suggest that it depends on the purpose of the experiments and type of blood vessels whether ring or spiral segments should be chosen as the isolated vascular tissue.