Ca2+ sparks are the elementary events of intracellular Ca2+ release from the sarcoplasmic reticulum in cardiac myocytes. In order to investigate whether spontaneous L-type Ca2+ channel activation contributes to the genesis of spontaneous Ca2+ sparks, we used confocal laser scanning microscopy and fluo-4 to visualize local Ca2+ sparks in intact rat ventricular myocytes. In the presence of 0.2 mmol/L CdCl2 which inhibits spontaneous L-type Ca2+ channel activation, the rate of occurrence of spontaneous Ca2+ sparks was halved from 4.20 to 2.04 events/(100 microm x s), with temporal and spatial properties of individual Ca2+ sparks unchanged. Analysis of the Cd2+-sensitive spark production revealed an open probability of approximately 10(-5) for L-type channels at the rest membrane potentials (-80 mV). Thus, infrequent and stochastic openings of sarcolemmal L-type Ca2+ channels in resting heart cells contribute significantly to the production of spontaneous Ca2+ sparks.