Kynurenic acid is an endogenous product of the tryptophan metabolism. Studies on the mechanism of its action have revealed that kynurenic acid at high concentrations is a competitive antagonist of the N-methyl-D-aspartate receptor and acts as a neuroprotectant in different neurological disorders. This in vitro investigation was designed to show that kynurenic acid acts differently at low concentrations. In vitro electrophysiological examinations on the young rat hippocampus confirmed the well-known finding that kynurenic acid in micromolar concentrations exerts an inhibitory effect. However, in nanomolar concentrations, kynurenic acid does not give rise to inhibition, but in fact facilitates the field excitatory postsynaptic potentials. The results available so far are compatible with the idea that kynurenic acid in the concentration range between a few hundred nanomolar and micromolar displays different effects. Its probable action on different receptors, inducing the different mechanisms, is discussed. The findings strongly suggest the neuromodulatory role of kynurenic acid under both physiological and pathological circumstances.