In synapses lacking a fast vesicular Ca2+-sensor from the synaptotagmin (syt) protein family, synchronous early release is absent while a later asynchronous component is preserved or enhanced. Sr2+ acting as a substitute for Ca2+ in triggering vesicle release has been shown to be less efficacious in promoting synchronous release than Ca2+ and to produce a more asynchronous from of release. We therefore asked, whether Sr2+ would be more or less efficient than Ca2+ in promoting the asynchronous release remnant in syt-deficient connections. Using high-resolution simultaneous pre- and postsynaptic whole cell patch-clamp ecordings from pairs of synaptically connected GABAergic neurons in cell culture, we determined that Sr2+ and Ca2+ are equally efficacious in promoting asynchronous release from syt-deficient presynaptic terminals. In addition, the degree of asynchrony of Sr2+- mediated exocytosis at control synapses was much less than that found both in Ca2+ and Sr2+ at syt-deficient connections. In summary, we conclude that Sr2+ normally activates syts and that a component of the relative asynchony of Sr2+-mediated release may be due to the fact that the Ca2+-sensor for asynchronous release lacks the abililty of syts to strongly differentiate between Sr2+ and Ca2+.