The effects of Mg++ 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% O2 and kept at 35℃. The results obtained were as follows: 1) In the uterine strips contracting spontaneously, as Mg++ concentration increased in the Tyrode solution the amplitude of peak tension decreased in all the experimental solutions containing the various concentrations of Ca++(0.5 ~ 4 mM). And the amplitude of peak tension increased in inverse proportion to the [Mg++]/[Ca++] ratio. It is suggested that the tension-lowering effect of Mg++ 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++]/[Ca++] ratio ratio increased up to 1/2, but the frequency decreased above this ratio. It is speculated that Mg++ would influence the excitability control action of Ca++.
