&NA;We examined vascular reactivity to various vasoconstrictors and dilators, and the changes in calcium‐calmodulin levels in canine basilar arteries after subarachnoid hemorrhage (SAH). Contractile responses to noradrenaline, serotonin, and potassium chloride were markedly attenuated at 48 hours (P< 0.05), and further attenuated at 7 and 14 days after SAH (P< 0.01). Dilation responses to calcium antagonist were maintained at 48 hours after SAH, but were markedly reduced at 7 and 14 days after SAH (P< 0.05). Transmission electron micrographs of the basilar artery showed contraction of the media between 48 hours and 7 days and degeneration of smooth muscle cells over the 7 days after SAH. Electron microscopic cytochemical examination for calcium showed that intracellular deposits of calcium pyroantimonate increased in smooth muscle cells of basilar arteries at 1 hour after the first intracisternal injection of blood (early spasm), but decreased in smooth muscle cells at 48 hours after SAH (at the beginning of delayed vasospasm). They decreased further in the vessels 7 days after SAH. The calmodulin contents in the basilar arteries were decreased slightly at 6 hours, and significantly (P< 0.05) at 48 hours after SAH, as determined by radioimmunoassay and phosphodiesterase assay. Therefore, it is considered that delayed vasospasm is not simply an active contraction of the vessels, but a functional or structural derangement of contractile elements of smooth muscle cells after 48 hours after SAH. (Neurosurgery25:753‐761, 1989)