Much has been discovered studying sarcoplasmic reticulum (SR) Ca release channels in SR vesicles and lipid bilayers. We have focused on how SR Ca release is regulated in intact mammalian ventricular myocytes, using fluorescent Ca indicators, voltage clamp, and confocal microscopy. Three major factors appear to contribute to the probability of spontaneous localized SR Ca release events (or Ca "sparks") in resting myocytes: (1) cytosolic [Ca], (2) SR Ca content, and (3) time after previous activity (i.e., recovery from adapted or inactivated state). These same three factors function during excitation-contraction (E-C) coupling and can explain rest potentiation of twitches, increased fractional SR Ca release at higher SR Ca loads, and Ca overload. Since SR Ca release is sensitive to both ICa and SR Ca load, we have controlled (and measured) these parameters. At constant SR Ca load and ICa in intact cells we have found that SR Ca release is increased by Ca-calmodulin-dependent protein kinase (CaMKII) and FK506 (which may interfere with the interaction between the Ca release channel and the FK binding protein) and is reduced by the Ca channel agonist Bay K 8644, CaMKII inhibitors, and during ventricular hypertrophy. Thus the regulation of the SR Ca release channel in the intact cell is an important factor in cellular cardiac function.