Expression of Syt1^{D229E.Scer\UAS} under the control of Scer\GAL4^elav.PLu in a Syt1^AD4 mutant background (i.e. where there is no endogenous Syt1) decreases the synchronous evoked release by more than 80%. The mean amplitude of the EJP is markedly decreased in these mutants. However, spontaneous release is unchanged, both in amplitude and frequency. The evoked response is also significantly reduced compared to controls in extracellular Ca[2+] concentrations ranging from 0.6 to 5 mM. However, the apparent Ca[2+] affinity of release is decreased in Syt1^{D229E.Scer\UAS} flies; 45% more Ca[2+] is required to trigger a half maximal response, compared to controls. The Ca[2+] cooperativity of release is not affected in these flies.