The effects of Mg<sup>++</sup> upon the spontaneous contraction activated by 1 IU/l oxytocin were studied in the isolated rat uterine muscle. Longitudinal muscle strips u¡¤ere prepared from the rat uteri at the estrous stage. All experiments were performed in tris-buffered Tyrode solution which was aerated with 100% O<sup>2</sup> and kept at 35℃. The results obtained were as follows: 1) In the uterine strips contracting spontaneously, as Mg<sup>++</sup> concentration increased in the Tyrode solution the amplitude of peak tension decreased in all the experimental solutions containing the various concentrations of Ca<sup>++</sup>(0.5 ~ 4 mM). And the amplitude of peak tension increased in inverse proportion to the [Mg<sup>++</sup>]/[Ca<sup>++</sup>] ratio. It is suggested that the tension-lowering effect of Mg<sup>++</sup> would be developed through decreasing intracellular ionized free calcium ion concentration by uncertain mechanism. 2) The frequency of the uterine contraction activated by oxytocin increased as the [Mg<sup>++</sup>]/[Ca<sup>++</sup>] ratio ratio increased up to 1/2, but the frequency decreased above this ratio. It is speculated that Mg<sup>++</sup> would influence the excitability control action of Ca<sup>++</sup>.