Synaptotagmin I (Syt-I) is a Ca2+-sensing protein for rapid synaptic vesicle exocytosis. The phenotype of Syt-I -/- synapses is characterized by a lack of early synchronous transmission, while an asynchronous mode, which is potentiated by tetanic stimulation, is intact. In wild-type synapses, equimolar substitution of Sr2+ for Ca2+ results in a marked decrease of the synchronous component of release with a strong increase in asynchronous/late release, suggesting that the release mechanisms for synchronous and asynchronous release might have differential sensitivities for the two divalent cations. We therefore asked whether Sr2+-substitution would affect asynchronous transmission remaining in the Syt-I -/- situation. In dual pre- and postsynaptic whole cell voltage-clamp recordings of pairs of neurones cultured from the striatum of Syt-I -/- mice at postnatal day 0, presynaptic stimulation with trains of 10 action potentials at 40 Hz resulted in a barrage of asynchronous quantal IPSCs lasting > 2 s. Amplitude and time-course of this asynchronous release were left unchanged by substitution of Sr2+ for Ca2+ at 2 mM. Total release measured as the response integral in Sr2+ was 98.1 +/­ 1.68 (SEM) % of that observed in Ca2+ (n=4, p>0.15). We conclude that the mechanism of asynchronous exocytosis operative in the Syt -/- synapses studied here, shows little or no selectivity for Ca2+ over Sr2+ or vice versa.