The Frank-Starling law of the heart (also known as Starling's law) states that when the diastolic filling of the heart is increased or decreased with a given volume, the volume of blood which is then ejected from the heart increases or decreases by the same amount. More blood in: more blood out.

Effectively, this means that the force of contraction will increase as the heart is filled with more blood and is a direct consequence of the effect of an increasing load on a single muscle fibre. The force that any single muscle fibre generates is proportional to the initial sarcomere length, and the stretch on the individual fibres is related to the end-diastolic volume of the ventricle. In the human heart, maximal force is generated with an initial sarcomere length of 2.6 micrometres, a length which is rarely exceeded in the normal heart.

This can be seen most dramatically in the case of a premature ventricular contraction. The premature ventricular contraction causes early emptying of the left ventricle (LV) into the aorta. Since the next ventricular contraction will come at its regular time, the filling time for the LV increases, causing an increased LV end diastolic volume. Because of the Frank-Starling law, the next ventricular contraction will be more forceful, causing the ejection of the larger than normal volume of blood, and bringing the LV end-systolic volume back to baseline.

The law is named after the two physiologists Otto Frank and Ernest Starling who first described it.