Selective activation of second messenger pathways were tested on presynaptic metabotropic glutamate receptor action at mossy fibre-CA3 synapses, using a rat hippocampal slice preparation. Application of the protein kinase C activator, phorbol 12,13-diacetate, or the adenylate cyclase activator, forskolin, markedly enhanced the mossy fibre field excitatory postsynaptic potentials, and suppressed the relative magnitude of the synaptic depression induced by (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine, an agonist at group-II metabotropic glutamate receptors. These effects were also observed in a low Ca2+ solution, suggesting that they were not due to saturation of transmitter release process. Inactive analogues of the respective activators (4alpha-phorbol 12,13-didecanoate and 1,9-dideoxyforskolin) neither enhanced the mossy fibre responses nor suppressed (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine-induced synaptic depression. These results suggest that the presynaptic inhibitory action of group-II metabotropic glutamate receptors at mossy fibre-CA3 synapses could be negatively regulated by protein kinase C- and cyclic AMP-dependent mechanisms.