The distribution of Ca2+ between mitochondrial and non-mitochondrial compartments within intact synaptosomes is investigated during the net Ca2+ uptake induced by plasma membrane depolarization. The steady-state synaptosomal Ca2+ content (5.8 +/- 0.3 nmol/mg protein) is increased by 77% by plasma depolarization induced by veratridine plus ouabain (9.7 +/- 0.6 nmol/mg protein) and by 100% by high K+ (50 mM) (11.0 +/- 0.9 nmol/mg protein). Prior abolition of the mitochondrial membrane potential, and hence inhibition of intrasynaptosomal mitochondrial Ca2+ accumulation, decreased the steady-state Ca2+ accumulation by 40% in both the control and the veratridine-ouabain depolarization, and by almost 60% in the case of high K+ depolarization. Similar values were obtained for the release of Ca2+ from synaptosomes when the mitochondrial membrane was depolarized after a steady state had been attained. Control experiments demonstrated that contaminating free mitochondria were not responsible for the altered Ca2+ accumulation. That the decrease in the Ca2+ accumulation on mitochondrial depolarization corresponds to the extent of the mitochondrial pool was confirmed by rapid synaptosomal disruption with digitonin which gave values of 2.5 +/- 0.5 nmol/mg protein, 4.4 +/- 0.9 nmol/mg protein and 6.9 nmol/mg protein for control or veratridine/ouabain- and high-[K+]-depolarized synaptosomes, respectively. The lesser contribution of intrasynaptosomal mitochondria during veratridine/ouabain-induced depolarization is proposed to be a consequence of raised cytosolic Na+ concentrations activating the mitochondrial Ca2+ efflux pathway. The results demonstrate that intrasynaptosomal mitochondria represent a metabolically responsive Ca2+ pool in situ.