The Ca2+-induced release of Ca2+ from he sarcoplasmic reticulum (SR) is not an all-or-none process but is graded with 1) the level of preload of the SR with Ca2+, 2) the rate of change of [free CA2+], and 3) the level of [free CA2+] used as a trigger. Among adult ventricular cardiac tissues the Ca2+-induced release of Ca2+ is absent in the frog ventricle, but present in avian and all mammalian ventricular tissues studied with large variations among species and maximum prominence in the rat ventricle. During the development of the rat ventricular cell there is a progressive maturation of the Ca2+-induced release of Ca2+, which is absent before birth. In the same animal species the Ca2+-induced release of Ca2+ is more developed in the atrium than in the ventricle. In the dog cardiac Purkinje tissue a higher [free Ca2+] preload and a higher [free Ca2+] trigger are required than for the dog ventricle. A working hypothesis is suggested according to which the mechanism of excitation-contraction coupling increases in complexity as the diameter of the cell increases, with maximum complexity presented by the fast skeletal muscle cell.