The main consequence of subarachnoid hemorrhage, for those who survive bleeding, is delayed, persistent vasospasm of intracranial conduit arteries which occurs between the third and seventh day after the insult and results in symptomatic brain ischemia in about 40% of cases. This vasospasm is considered to be a major cause of disability of post-SAH patients. Despite extensive experimental and clinical research, mechanisms of vasospasm are not fully understood. Dysfunction of the endothelium resulting in enhanced production of vasoconstrictors, phenotypic changes of the receptors in endothelium and smooth muscle cells, increased sensitivity of vascular smooth muscle cells to vasoconstrictors, release of spasmogens from lysed blood clot and inflammatory response of the vascular wall have been demonstrated and discussed as pathological mechanisms participating in the development of spasm. In recent years more attention is paid to the functional and structural changes in microcirculation and a concept of microvascular spasm is evolving. Our experimental studies in rat model of SAH strongly suggest that microcirculatory dysfunction and delayed vasospasm are related to the severity of acute, transient ischemia caused by critical decrease of perfusion pressure and active vasoconstriction immediately after the bleeding.