We examined the effects of activation of metabotropic glutamate receptors (mGluRs) on glutamatergic synaptic transmission at the neuromuscular junction of newly hatched Drosophila larvae. In nominally Ca(2+)-free solutions puff-application of low concentrations of glutamate evoked a transient frequency increase of miniature synaptic currents (mSCs). The mean amplitude of mSCs was unaffected, suggesting that this effect was presynaptic. Similar alterations of the mSC frequency were obtained using the mGluR agonists, (S)-4C3HPG, DCG-IV, or (1S,3S)-ACPD, but not when using agonists for ionotropic glutamate receptors, NMDA, AMPA or kainate. An mGluR antagonist, MCCG-I, blocked the effect of agonists on the mSC frequency. An adenylate cyclase activator, forskolin, and a cAMP analog, CPT-cAMP, mimicked the effects of mGluR activation. Meanwhile, an adenylate cyclase inhibitor, SQ22,536, blocked the mGluR agonist-induced effects, and in rutabaga, an adenylate-cyclase-defective mutant, the effect of the agonist was greatly reduced. In the presence of external Ca2+, (S)-4C3HPG decreased the failure rate and increased the mean amplitude of stimulus-evoked SCs, while MCCG-I decreased the amplitudes. We suggest that at the larval Drosophila neuromuscular junction endogenous glutamate released at the terminal potentiates synaptic transmission via a process involving cAMP.