K-stimulated (voltage-dependent) 45Ca uptake in rat brain synaptosomes was measured at times ranging from 0.1 to 10 s, in experiments that employed a rapid-mixing device to initiate and terminate radiotracer uptake. The rapid mixing did not disrupt the functional integrity of the synaptosomes, as judged by their ability to take up Ca. In solutions containing a low (0.02 mM) concentration of Ca, the rate of K-stimulated Ca uptake measured after 0-0.12 s depolarization was 8 times greater than that measured after 5-10 s of depolarization. The decline in rate of K-stimulated Ca uptake was not due to tracer backflux from the synaptosomes, nor to Ca loading of the nerve terminals, since it also occurred after synaptosomes were depolarized in solutions without Ca. It is suggested that this decline in rate of Ca uptake after depolarization was due to inactivation of voltage-dependent Ca channels in the nerve terminals. This inactivation appeared to be voltage rather than Ca dependent. The extent to which K-stimulated Ca uptake declined after depolarization in high-K solution depended on the K concentration that was used to depolarize the synaptosomes. Whereas pre-incubation in solution with one-half of the Na replaced by K significantly reduced subsequent K-stimulated Ca uptake, pre-incubation in non-depolarizing solution, with one-half of the Na replaced by choline, had no significant effect on subsequent K-stimulated Ca uptake. In solutions containing a high (0.5-2 mM) concentration of Ca, the rate of K-stimulated Ca uptake measured after 0-0.12 s was 40 times greater than that measured after 5-10 s. High Ca accelerated the rate at which K-stimulated Ca uptake declined with prolonged depolarization. The effect was mimicked by high (10 mM) concentrations of Sr, but not of Ba. The accelerated rate of decline observed with high Ca could be either a direct effect of Ca on the Ca channels or, more probably, an indirect effect of Ca loading on the nerve terminals. The apparent efficacy of several Ca-channel blockers (Ni, La and verapamil) in reducing K-stimulated Ca uptake was enhanced when the synaptosomes were depolarized in the presence of inhibitory agents for brief (less than 1 s) intervals before K-stimulated Ca uptake was measured.