Ca(2+) sparks monitor transient local releases of Ca(2+) from the sarcoplasmic reticulum (SR) into the myoplasm. The release takes place through ryanodine receptors (RYRs), the Ca(2+)-release channels of the SR. In intact fibers from frog skeletal muscle, the temporal and spatial properties of voltage-activated Ca(2+) sparks are well simulated by a model that assumes that the Ca(2+) flux underlying a spark is 2.5 pA (units of Ca(2+) current) for 4.6 ms (18 degrees C). This flux amplitude suggests that 1-5 active RYRs participate in the generation of a typical voltage-activated spark under physiological conditions. A major goal of future experiments is to estimate this number more precisely and, if it is two or more, to investigate the communication mechanism that allows multiple RYRs to be co-activated in a rapid but self-limited fashion.