Mice carrying a mutation in the synaptotagmin I gene were generated by homologous recombination. Mutant mice are phenotypically normal as heterozygotes, but die within 48 hrafter birth as homozygotes. Studies of hippocampal neurons cultured from homozygous mutant mice reveal that synaptic transmission is severely impaired. The synchronous, fast component of Ca2+-dependent neurotransmitter release is decreased, whereas asynchronous release processes, including spontaneous synaptic activity (miniature excitatory postsynaptic current frequency) and release triggered by hypertonic solution or a-latrotoxin, are unaffected. Our findings demonstrate that synaptotagmin I function is required for Ca2+ triggering of synchronous neurotransmitter release, but is not essential for asynchronous or Ca*+-independent release. We propose that synaptotagmin I is the major low affinity Ca2+ sensor mediating Ca*+ regulation of synchronous neurotransmitter release in hippocampal neurons.