Previous studies suggest that the vesicular cysteine-string protein (CSP) may modulate presynaptic Ca(2+) channel activity in fast neurotransmitter release. To test this hypothesis, we analyzed the dynamics of presynaptic Ca(2+) ion influx with the Ca(2+) indicator fluo-4 AM at csp mutant neuromuscular junctions of Drosophila. From 24 to 30 degrees C, stimulus-evoked, relative presynaptic Ca(2+) signals were increasingly larger in csp mutant boutons than in controls. Above 30 degrees C, Ca(2+) signals declined and were similar to controls at 34 degrees C. A prolonged decay of Ca(2+) signals in mutant boutons at high temperatures indicated abnormally slow Ca(2+) clearance. Cytosolic Ca(2+) at rest was determined with the ratiometric Ca(2+) indicator fura-2 AM and was similar in mutant and control boutons at 24 degrees C but higher in mutant boutons at 34 degrees C. Despite larger Ca(2+) signals in mutant boutons, evoked neurotransmitter release was always reduced in csp mutants and exhibited pronounced facilitation. Thus, a lack of Ca(2+) entry cannot explain the reduction of neurotransmitter release in csp mutants. At all temperatures tested, raising extracellular Ca(2+) increased transmitter release elicited by single stimuli in csp mutants. Collectively, these data suggest multiple functions for CSP at synaptic terminals. Increased Ca(2+) signals coupled with reduced release suggest a direct function of CSP in exocytosis downstream from Ca(2+) entry. Because the reduction of evoked release in csp mutants is counteracted by increased Ca(2+) levels, we suggest that CSP primarily increases the Ca(2+) sensitivity of the exocytotic machinery.