That an increase of myoplasmic free Ca2+ concentration ([free Ca2+]) pre cedes contraction has been shown in skeletal muscle cells injected with various Ca2+-sensitive probes, including aequorin (9). For approximately ten years Blinks maintained that no steadfast reason precluded such a demonstration in heart muscle (eg 8). During these ten years he and his collaborators carefully characterized the relation between [free Ca2+] and aequorin bioluminescence and applied this method to progressively smaller cells (8, 9). It is encouraging to know that he and Allen have finally succeeded in obtaining records of aequorin bioluminescence from multicel lular preparations of cardiac muscle (2). This exemplifies the patient experi mental work required for progress in the complex field of cardiac excitation-contraction coupling and explains why this field has progressed slowly since the time of the excellent review by Fozzard (22). Recent studies on the role of the sarcolemma and the sarcoplasmic reticulum (SR) in the beat-to-beat Ca2+ regulation and of other organelles in slower Ca2+ move ments are discussed here in order to indicate some of the alternative hypoth eses for the mechanism of cardiac excitation-contraction coupling that are currently being tested.[See (53) for a justification of this approach.]