Stimulus-secretion coupling was monitored with capacitance detection in bovine chromaffin cells recorded in perforated patch mode and stimulated with trains of depolarizing pulses. A subset of stimulus trains evoked a response with a Ca 2ϩ -exocytosis relationship identical to that obtained for single depolarizing pulses (Engisch and Nowycky, 1996). Other trains evoked responses with enhanced or diminished Ca 2ϩ efficacy relative to this input-output function. The probability of obtaining a particular Ca 2ϩ -exocytosis relationship was correlated with the amount of Ca 2ϩ entry per pulse, such that shorter pulses or smaller currents were associated with the greatest efficacy, and longer pulses and larger currents with the lowest efficacy.Apparent enhancements in Ca 2ϩ efficacy were not caused by residual Ca 2ϩ summing between pulses, because decreasing the interval between pulses usually reduced efficacy in the same cell; conversely, increasing the interval between pulses did not prevent an enhanced Ca 2ϩ -exocytosis relationship. Apparent decreases in Ca 2ϩ efficacy were not caused by depletion of an available pool of release-ready vesicles, because an equivalent amount of total Ca 2ϩ entry during a single long depolarizing pulse usually evoked a much larger secretory response in the same cell. Finally, there were no striking differences in global Ca 2ϩ levels monitored with the fluorescent indicator Fura Red that could account for apparent changes in Ca 2ϩ efficacy during repetitive stimulus protocols. It appears that in chromaffin cells, the Ca 2ϩ -exocytosis relationship is subject to activity-dependent changes during a stimulus train and can be modulated up or down from a basal state accessed by single pulse stimulations.