The K +-evoked release of dynorphin A(1–8)-like immunoreactivity from guinea pig hippocampal mossy fiber synaptosomes was inhibited 53% by L(+)-2-amino-4-phosphonobutyrate ( L(+)APB, 300 μM), a glutamate analogue. Equimolar L(+)APB also inhibited the Ca 2+-dependent component of endogenous L-glutamate reelase from these mossy fiber synaptosomes by 40%. The K +-evoked release of both glutamate and dynorphin A(1–8) from rat hippocampal mossy fiber synaptosomes were unaffected by L(+)APB. It is proposed that L(+)APB selectivity suppresses the excitatory mossy fiber synaptic input by inhibiting the Ca 2+-dependent release of glutamate and dynorphin A(1–8) from guinea pig but not rat hippocampal mossy fiber terminals.