We investigated mechanisms involved in the modulation of long-term potentiation by low concentrations of N-methyl-D-aspartate in the CA1 region of rat hippocampal slices. When applied for 5 min prior to and during tetanic stimulation, 1 microM N-methyl-D-aspartate inhibited long-term potentiation induction. Studies examining paired-pulse facilitation of non-N-methyl-D-aspartate receptor-mediated synaptic responses suggest that the effects of N-methyl-D-aspartate result in part from a presynaptic mechanism. This conclusion is supported by the observation that 1 microM N-methyl-D-aspartate failed to diminish N-methyl-D-aspartate receptor-mediated synaptic currents and that agents that enhance glutamate release, including high extracellular concentrations of calcium and an adenosine A1 receptor antagonist, overcome the long-term potentiation inhibition. Furthermore, the calcineurin inhibitors, FK-506 and cyclosporin A, as well as the phosphatase 1 and 2A inhibitor, okadaic acid, blocked the effects of N-methyl-D-aspartate on long-term potentiation suggesting a role for phosphatase activation in modulating the induction of long-term potentiation. These results show that the inhibition of long-term potentiation by untimely N-methyl-D-aspartate receptor activation is reversed by treatments that enhance glutamate release and suggest that adenosine release and diminished calcium influx during tetanic stimulation coupled with phosphatase activation contribute to the modulation of synaptic plasticity.