1. We have investigated the possible role of frequenin in the modulation of synaptic facilitation at the larval Drosophila neuromuscular junctions. Excitatory junctional currents (EJCs) and presynaptic nerve terminal currents were recorded by external electrodes in normal larvae and in transgenic larvae carrying an extra insertion of the frequenin cDNA. 2. Motor nerve stimulation by twin pulses or trains of stimuli provoked EJC facilitation which was about three times higher in transgenic larvae compared to controls. Unconditioned EJCs revealed, however, similar quantal content and Ca2+ sensitivity in both Drosophila strains. 3. Differences between normal and transgenic Drosophila in the quantal content of the facilitated EJC do not depend on differences in the duration of the repolarization phase of the presynaptic action potential. 4. Perfusion of tetrodotoxin or of low-Na+ solutions abolished the enhancement of the EJC facilitation observed in the transformants. These treatments only slightly affected the facilitation of normal junctions. 5. These results suggest that (i) internal Na+ accumulation can enhance facilitation of transmitter release in Drosophila neuromuscular junctions overexpressing frequenin, and (ii) this effect possibly depends on a modulation of the activity of the Na(+)-Ca2+ exchanger by frequenin.