Effects of BDNF (50-100 ng/ml) on long-term potentiation (LTP) and depression (LTD) were studied in rat hippocampal slices. BDNF did not affect the basal level of synaptic transmission nor LTP induced by a strong tetanus but facilitated the induction of LTP with a mild tetanus. BDNF attenuated the level of LTD. These effects of BDNF were blocked by K252a or U73122, suggesting the mediation of Trk B receptors and phospholipase C. Patch clamp analysis revealed that BDNF selectively inhibited the inhibitory synaptic current without affecting the excitatory synaptic current. Therefore, the effects of BDNF were similar to those of GABA A receptor antagonist. However, analysis of frequency-response relationship of synaptic transmission showed distinct differences. Effects of BDNF on spatial learning were investigated using water maze task. Daily injection of BDNF significantly decreased the escape latency in fimbria-fornix-lesioned animals without affecting choline acetyltransferase activities in the hippocampus. Taken together, these results suggest that BDNF improves the learning disorder by facilitating the synaptic plasticity in the hippocampus.