The effect of irrigation with artificial cerebrospinal fluid (CSF) containing various magnesium ion (Mg²⁺) concentrations on vasospastic arteries was investigated in the dog. Cerebral vasospasm was induced by the experimental subarachnoid hemorrhage model in 15 beagle dogs. Cisternal irrigation was performed for 1 hour via a microcatheter placed in the cisterna magna with commercially available artificial CSF (ARTCEREB®) with physiological concentration of Mg²⁺ (2.2 mEq/l) (ACM group, n = 5), ARTCEREB solution without Mg²⁺ (ACR group, n = 5), and ARTCEREB solution with higher Mg²⁺ concentration (5 mEq/l) (ACMM group, n = 5). CSF electrolyte concentrations and the diameters of the basilar and vertebral arteries were measured. In the ACM group, no changes were detected in either CSF Mg²⁺ concentration or arterial diameters. In the ACR group, the CSF Mg²⁺ decreased significantly to 0.8 ± 0.07 mEq/l from the baseline value of 1.4 ± 0.03 mEq/l, and both basilar and vertebral artery diameters were significantly decreased to 0.61 ± 0.18 mm and 0.57 ± 0.23 mm from their baseline values of 0.74 ± 0.22 mm and 0.68 ± 0.17 mm, respectively. In the ACMM group, the CSF Mg²⁺ significantly increased to 2.4 ± 0.15 mEq/l from the baseline value of 1.4 ± 0.05 mEq/l, and both basilar and vertebral artery diameters were significantly increased to 0.84 ± 0.19 mm and 0.90 ± 0.22 mm from their baseline values of 0.71 ± 0.21 mm and 0.69 ± 0.24 mm, respectively. Irrigation with artificial CSF solution without Mg²⁺ causes vasoconstriction of the cerebral artery. Irrigation with artificial CSF with appropriate Mg²⁺ concentration is essential, especially in patients with subarachnoid hemorrhage.