The time course of Ca2+-dependent [3H]acetylcholine [( 3H]ACh) release and inactivation of 45Ca2+ entry were examined in rat brain synaptosomes depolarized by 45 mM [K+]0. Under conditions where the intrasynaptosomal stores of releasable [3H]ACh were neither exhausted nor replenished in the course of stimulation, the K+-evoked release consisted of a major (40% of the releasable [3H]ACh pool), rapidly terminating phase (t1/2 = 17.8 s), and a subsequent, slow efflux that could be detected only during a prolonged, maintained depolarization. The time course of inactivation of K+-stimulated Ca2+ entry suggests the presence of fast-inactivating, slow-inactivating, and noninactivating, or very slowly inactivating, components. The fast-inactivating component of the K+-stimulated Ca2+ entry into synaptosomes appears to be responsible for the rapidly terminating phase of transmitter release during the first 60 s of K+ stimulus. The noninactivating Ca2+ entry may account for the slow phase of transmitter release. These results indicate that under conditions of maintained depolarization of synaptosomes by high [K+]0 the time course and the amount of transmitter released may be a function of the kinetics of inactivation of the voltage-dependent Ca channels.