In crayfish motor nerve terminals L-glutamate (Glu) is the excitatory transmitter and low L-Glu concentrations exert autoinhibition by inhibiting release of Glu quanta from the terminals. This autoinhibition has been shown to be mediated by binding and transport of L-Glu by Glu transporters in the presynaptic membrane. Activated transporters open an associated Cl− channel and inhibit release [J. Dudel & M. Schramm (2003) Eur. J. Neurosci., 18, 902–910]. The excitatory, glutamatergic synaptic transmission is specific for the L-Glu isomer. However, transporters are non-selective for the stereoisomers. It is shown here that low concentrations (5 μM) of D- as well as L-Glu inhibit quantal release on average to 55 and 68%, respectively. The power of inhibition varies widely at different terminals but the local sensitivity to D-Glu is seen to be the same as that for L-Glu. L-Glutamate has been reported to reduce the mean amplitude of nerve terminal action currents (excitatory nerve terminal currents) by about 10%, presumably due to the opening of Cl− channels. Evidence is given that D-Glu also inhibits this by an average of 10% (P < 0.001), as expected if both L- and D-Glu activate a transporter-associated Cl− conductance. The results give further support for this novel mechanism of regulation of synaptic strength.