Submembrane [Ca2+]i changes were examined in rat chromaffin cells by monitoring the activity of an endogenous Ca(2+)-dependent protein: the large conductance Ca(2+)-and voltage-activated K+ channel (also known as the BK channel). The Ca2+ and voltage dependence of BK current inactivation and conductance were calibrated first by using defined [Ca2+]i salines. This information was used to examine submembrane [Ca2+]i elevations arising out of Ca2+ influx and muscarine-mediated release of Ca2+ from intracellular stores. During Ca2+ influx, some BK channels are exposed to [Ca2+]i of at least 60 microM. However, the distribution of this [Ca2+]i elevation is highly nonuniform so that the average [Ca2+]i detected when all BK channels are activated is only approximately 10 microM. Intracellular dialysis with 1 mM or higher EGTA spares only the BK channels activated by the highest [Ca2+]i during influx, whereas dialysis with 1 mM or higher BAPTA blocks activation of all BK channels. Submembrane [Ca2+]i elevations fall rapidly after termination of short (5 msec) Ca2+ influx steps but persist above 1 microM for several hundred milliseconds after termination of long (200 msec) influx steps. In contrast to influx, the submembrane [Ca2+]i elevations produced by release of intracellular Ca2+ by muscarinic actetylcholine receptor (mAChR) activation are much more uniform and reach peak levels of 3-5 microM. Our results suggest that during normal action potential activity only 10-20% of BK channels in each chromaffin cell see sufficient [Ca2+]i to be activated.