The effect of long-term potentiation (LTP) on endogenous amino acid release from rat hippocampus slices was studied. LTP was induced in vivo by application of a tetanus (200 Hz, 200 ms) to the Schaffer collateral fibers in unanesthetized rats. Endogenous release of glutamate and gamma-aminobutyric acid (GABA) was investigated 60 min after tetanization in CA1 subslices of potentiated and control rats. No significant effects of LTP were observed in basal and K(+)-induced Ca(2+)-independent release components of these amino acids. In contrast, K(+)-induced Ca(2+)-dependent release of both glutamate and GABA increased approximately 100% in slices from potentiated rats. No differences were observed in total content of glutamate and GABA between the subslices from control and LTP animals. These results suggest a persistent increase in the recruitment of the presynaptic vesicular pool of glutamate and GABA during LTP.