Subarachnoid hemorrhage (SAH)-induced cerebral vasospasm causes serious neurological morbidity and mortality mainly because of the absence of effective treatment. Therefore, we reviewed the molecular mechanisms involved in the development of the cerebral vasospasm based on the experimental data in the two-hemorrhage canine model. The characteristic feature of vasospasm is a continuous elevation of intracellular Ca2+ levels in the cerebral artery, as indicated by the continuous activation of μ-calpain and Ca2+ / calmodulin-dependent myosin light chain kinase (MLCK) phosphorylation of the myosin light chain. In contrast, KCl- or serotonin-induced vasocontraction displays a transient increase in Ca 2+ concentration. The elevation of intracellular Ca2+ levels in vasospasm is induced through enhanced Ca2+ release from the sarcoplasmic reticulum and influx from the extracellular space by the activation of tyrosine kinase pathway and also probably by the proteolysis of Ca2+ channel by μ--calpain. Topical application of L-type Ca2+ channel blockers, ethylene-glycol-bis(β-aminoethylether)N,N-tetraacetic acid, genistein, calpeptin (a selective inhibitor of calpain), or ML-9 (a selective inhibitor of MLCK) induces the reversal of vasospasm probably as a result of a decrease in intracellular Ca 2+ levels mainly due to a reduction of Ca2+ influx by these three inhibitors. Rho-kinase is also activated during vasospasm. It inhibits myosin phosphatase through phosphorylation at the myosin phosphatase target subunit 1 and also probably through phosphorylation of the 17-kDa smooth muscle-specific myosin phosphatase inhibitor (CPI-17) to bring about Ca2+-independent vasospasm. This interpretation is supported by the reversal of vasospasm with Y-27632, a specific inhibitor of Rho-kinase. Arachidonic acid produced during vasospasm might inhibit myosin phosphatase probably directly and via activation of Rho-kinase or atypical protein kinase C (PKC). PKC activated during vasospasm may inhibit myosin phosphates directly and by phosphorylating CPI-17. The protein levels of thin filament-associated proteins, calponin and caldesmon, are decreased in vasospasm, whereas their phosphorylation levels are increased. Both changes probably contribute to the enhancement of vascular smooth muscle contractility. Furthermore, contractile and cytoskeletal proteins appear to be degraded in vasospasm probably by proteolysis with μ--calpain, suggesting that degradation of the structural and functional mechanisms related to smooth muscle contraction occurs. Thus, the mechanisms responsible for the development of cerebral vasospasm are complicated, but the prevention of intracellular Ca2+ elevation induced by SAH may not activate MLCK, calpain and PKC to largely suppressing the development of vasospasm.