Sodium azide (NaN3) and dinitrophenol (DNP) at low concentrations caused "Ca reversal", i.e. inhibition of oxytocin-induced tonic contraction of estrogen-dominated rat uterine smooth muscle in Ca-free solution. This inhibition was not due to depletion of ATP by NaN3 or DNP. Higher concentrations of NaN3 and DNP caused additional contraction. NaN3 and DNP dose-dependently released Ca ion from mitochondria isolated from the estrogen-dominated rat uterine smooth muscle in vitro. Electron microscopic studies have shown that in estrogen-dominated rat uterine smooth muscle cells, cytoplasmic membranes proliferate, resulting in compartmentalization of the myofilament-sarcoplasmic system and its separation from the receptor-effector system in the surface folds of the plasma membranes, which also contain some mitochondria. It is proposed that low concentrations of NaN3 and DNP release a small amount of Ca ion from these outer mitochondria and this Ca ion acts on the intracellular "site of Ca reversal" to induce reversal, i.e. inhibition. Higher concentrations of NaN3 and DNP are proposed to release a large amount of Ca ion from the central mitochondria near myofilaments and so induce contraction. The "site of Ca reversal" was shown to be intracellular as our previous postulation.