Many factors appear to control uterine activity in the human, either by direct action or by modulating the effects of other agents. There is no evidence to conclude that any one substance is the pre-eminent controller of physiological activity. The final common mediator of contraction evoked by stimulants is calcium and without this contractility does not occur. The action of many relaxant and contracting drugs can be ascribed to their effects on calcium binding and intracellular availability.