Dose-response relationships for Mg2+ in a Mg2+-free bathing solution were obtained with isolated canine femoral arteries. At concentrations less than 5 mM, relaxant responses appeared in the preparations. In a concentration range of 10-20 mM, however, every one of the preparations invariably showed dose-dependent contractions. The relaxant and contractile responses were induced by the action of Mg2+ per se but not by the hyperosmolarity associated with the addition of Mg2+. Neither response was affected by the drugs that inhibited the actions of catecholamine, acetylcholine, serotonin, or histamine. The contractile response was kept unchanged in Ca2+-free Locke's solution and was almost independent of environmental calcium concentrations ranging from 2.2 to 8.2 mM. The contractile response was not affected under the influence of Ca antagonists, verapamil, or Mn2+. The presence of Ba2+ or Sr2+ enhanced the relaxant response and dose dependently suppressed the contractile response. The contractile response was still evoked in smooth muscles that were in the state of K+ contracture. These results suggest the possibility that the contractile action of Mg2+ was mediated by a release of intracellular Ca2+ or exerted directly on the contractile machinery of arterial smooth muscle.