The relationship between ventricular wall tension and intraventricular blood pressure is examined from the point of view of a system which is accelerating. The usual relationship between these variables, often called the law of Laplace, is derived under assumptions of static equilibrium. In this paper the dynamic relationship is developed from basic physical theory employing the concept of centre of mass, and is used to determine the actual forces present on a moving equatorial strip of heart muscle. It is shown that even in cases of heightened ventricular activity, Laplace's law is a good approximation to the true situation. The significance of ventricular acceleration is discussed.