Arteries that develop vasospasm after subarachnoid hemorrhage (SAH) may have altered contractility and compliance. Whether these changes are due to alterations in the smooth muscle cells or the arterial wall extracellular matrix is unknown. This study elucidated the location of such changes and determined the calcium sensitivity of vasospastic arteries. Dogs were placed under general anesthesia and underwent creation of SAH using the double hemorrhage model. Vasospasm was assessed by angiography performed before and 4, 7 or 21 days after SAH. Basilar arteries were excised from SAH or control dogs (n=8-52 arterial rings from 2-9 dogs per measurement) and studied under isometric tension in vitro before and after permeabilization of smooth muscle with -toxin. Endothelium was removed from all arteries. Vasospastic arteries demonstrated significantly reduced contractility to KCl with a shift in the EC₅₀ towards reduced sensitivity to KCl 4 and 7 days after SAH (p<0.05, ANOVA). There was reduced compliance that persisted after permeabilization (p<0.05, ANOVA). Calcium sensitivity was decreased during vasospasm 4 and 7 days after SAH, as assessed in permeabilized arteries and in those contracted with Bay K 8644 in the presence of different concentrations of extracellular calcium (p<0.05, ANOVA). Depolymerization of actin with cytochalasin D abolished contractions to KCl but failed to alter arterial compliance. In conclusion, it is shown for the first time that calcium sensitivity is decreased during vasospasm after SAH in dogs, suggesting that other mechanisms are involved in maintaining the contraction. Reduced compliance seems to be due to an alteration in the arterial wall extracellullar matrix rather than the smooth muscle cells themselves since it cannot be alleviated by depolymerization of smooth muscle actin.