Intracellular Ca management regulates the force of cardiac contraction. In comparison to endotherms, ectothermic hearts are generally strongly dependent on extracellular Ca for contraction suggesting this type of excitation- contraction (e-c) coupling might represent the primitive vertebrate mode of cardiac activation. This study tests the hypothesis that intracellular Ca stores of the sarcoplasmic reticulum (SR) play a minor role in cardiac e-c coupling of a basal vertebrate, the European river lamprey (Lampetra fluviatilis). Contribution of SR to cardiac Ca management was examined by measuring ryanodine (Ry) sensitivity of cardiac contraction, determining the number of cardiac Ry receptors (RyRs) and their Ca sensitivity, assessing the size of SR Ca stores in enzymatically isolated cardiac myocytes and qualitative electron microscopic observation of SR structures. An extensive network of nonjunctional SR around myofibrils and numerous junctional SR couplings at the periphery of the myocytes was present in atrial and ventricular muscle of the lamprey heart. High numbers of Ca- sensitive RyRs were present in atrial and ventricular membranes, and 10 mM Ry inhibited large part of contraction force in atrial (37.8 ± 5.7 %) and ventricular (49.2 ± 6.5 %) muscle. SR is well-developed in the lamprey heart and plays a significant role in cardiac Ca management. Functionally this trait might be associated with high cardiac output and active predatory lifestyle of the lamprey.