Net and radioactive calcium movements were studied in the rat uterus during stimulation with acetylcholine and high potassium solutions. High potassium did not affect the efflux of intracellular Ca45, but was able to release Ca45 from a small parallel Ca fraction which was believed to be located in the cell membranes. High potassium did markedly slow the influx of Ca45 and caused a net calcium efflux. Acetylcholine had no effect on calcium movements in polarized myometrium, but it increased the Ca45 influx in depolarized uteri. Ca45 taken up during contraction exchanged more slowly during subsequent efflux than Ca45 taken up at rest. The results were interpreted as supporting the hypothesis that myometrial contraction is induced by a release of calcium from the inside of the cell membrane and the endoplasmic reticulum, and relaxation follows the removal of ionic cytoplasmic calcium by these same structures.