The effects of various concentrations of extracellular Mg2+ on electrical and mechanical activity of the rat portal vein were studied. The integrated spontaneous contractile activity in the preparation was largest in Mg2+-free solution, decreased to about 50% at 1.2 mM Mg2+ and was almost completely abolished at 10 mM Mg2+. Each spontaneous contraction became smaller whereas contraction frequency was less affected. Sucrose gap recordings showed that the reduced spontaneous mechanical output was associated with decreased electrical activity on increasing [Mg2+]0. Increasing [K+]0 from 6 to 12 mM normalized the spontaneous mechanical activity in 10 mM Mg2+ solution. Local registration of electrical activity with 3 extracellular glass capillary electrodes showed that inactive areas developed at the high Mg2+ concentrations. These findings indicate that Mg2+ exerts a hyperpolarizing action on the smooth muscle cell membrane and, at the highest concentration, interferes with intercellular propagation. [Mg2+]0 in the range of 0-10 mM had no effect on the amplitude of K+ (122 mM) contractures at [Ca2+]0 greater than 0.5 mM. At [Ca2+]0 less than or equal to 0.5 the amplitude diminished with increasing [Mg2+]0. The latter observation indicates that Mg2+ can interfere with the Ca2+-permeability of the depolarized cell membrane. 1 mM EDTA in Na-tris 0 mM Mg2+ and 0 mM Ca2+ seemed to lower intracellular Mg2+ below the minimum concentration needed for contractile activity.