Uptake and efflux of 89Sr in pregnant rat myometrium were not different from uptake and efflux of 45Ca although tension studies indicated that Sr2+ does not become sequestered. The effects of Ca+-free solution on uterine responses to acetylcholine (ACh) and K+ suggest that at 37 °C, these agents cause release of calcium from the same pool. Substitution of Na+ by Li+ in the medium caused a contraction which was dependent upon extracellular Ca2+, and which was accompanied by increased 45Ca uptake and increased Ca2+ content. Three inhibitory drugs lanthanum, SKF 525-A, and chlorpromazine inhibited responses of myometrium to ACh and K+, but did not demonstrate selective inhibition of one of these stimulants. Effects of these inhibitory drugs were at least partially due to decreased frequency of action potential discharge. Of these three inhibitory agents only lanthanum inhibited (partially) 45Ca and 89Sr uptake into myometrium.The lanthanum technique for measurement of cellular 45Ca influx failed to demonstrate differences in pools of calcium utilized for ACh and K+ induced contractions. The effects of lanthanum in contractility and 45Ca experiments are discussed.