We investigated how inhibition of mitochondrial Ca2+ uptake affects transmitter release from mouse motor terminals during brief trains of action potentials (500 at 50 Hz) in physiological bath [Ca2+]. When mitochondrial Ca2+ uptake was inhibited by depolarizing mitochondria with antimycin A1 or carbonyl cyanide m-chlorophenyl-hydrazone, the stimulation-induced increase in cytosolic [Ca2+] was greater (> 10 microM, compared to < or = 1 microM in control solution), the quantal content of the endplate potential (EPP) depressed more rapidly (approximately 84 % depression compared to approximately 8 % in controls), and asynchronous release during the stimulus train reached higher frequencies (peak rates of approximately 6000 s-1 compared to approximately 75 s-1 in controls). These effects of mitochondrial depolarization were not accompanied by a significant change in EPP quantal content or the rate of asynchronous release during 1 Hz stimulation, and were not seen in oligomycin, which blocks mitochondrial ATP synthesis without depolarizing mitochondria. Inhibition of endoplasmic reticular Ca2+ uptake with cyclopiazonic acid also had little effect on stimulation-induced changes in cytosolic [Ca2+] or EPP amplitude. We hypothesize that the high rate of asynchronous release evoked by stimulation during mitochondrial depolarization was produced by the elevation of cytosolic [Ca2+], and contributed to the accelerated depression of phasic release by reducing the availability of releasable vesicles. During mitochondrial depolarization, the post-tetanic potentiation of the EPP observed under control conditions was replaced by a post-tetanic depression with a slow time course of recovery. Thus, mitochondrial Ca2+ uptake is essential for sustaining phasic release, and thus neuromuscular transmission, during and following tetanic stimulation.