Large inward currents (tens of nanoamperes) were induced in voltage-clamped muscle fibres of blowfly larvae (Calliphora vicina) by inhibition of glutamate (Glu) uptake by means of substitution of sodium by lithium in the perfusion, or reduction of temperature from 23 to 10 degrees C. These currents appeared to be due to accumulation of (non-quantal) Glu in the synaptic cleft: (1) the amplitude of the current depended on desensitization to Glu, as it could be enhanced by concanavalin A pretreatment; (2) the current was inhibited by a specific blocker of Glu-activated channels, argiopin; (3) the current presented a power spectrum which was identical to the spectrum induced by glutamate application. It was shown the non-quantal release is not compensated by Glu uptake and under normal conditions (no lithium, room temperature) there was some Glu in the synaptic cleft, whose concentration was sufficient to induce significant current noise.