The properties of the Ca stores in the skinned fiber bundles of the guinea-pig taenia caeci were studied by a novel method to measure Ca released from the stores using a fluorescent Ca indicator, fura-2. It is found that the capacity of the Ca stores amounts some 170 mumoles/liter cell water. There are at least two Ca release mechanisms, Ca-induced Ca release (CICR) and Ca release induced by inositol-1,4,5-trisphosphate (IP3). Only about a half of the Ca is released by the Ca-induced Ca release, while IP3 releases most of the Ca. The rate of the IP3-induced Ca release is found to be strongly enhanced by submicromolar concentrations of Ca2+.