Brain-derived neurotrophic factor (BDNF) and its receptor TrkB regulate both short-term synaptic functions and long-term potentiation (LTP) of brain synapses, raising the possibility that BDNF/TrkB may be involved in cognitive functions. We have generated conditionally gene targeted mice in which the knockout of the trkB gene is restricted to the forebrain and occurs only during postnatal development. Adult mutant mice show increasingly impaired learning behavior or inappropriate coping responses when facing complex and/or stressful learning paradigms but succeed in simple passive avoidance learning. Homozygous mutants show impaired LTP at CA1 hippocampal synapses. Interestingly, heterozygotes show a partial but substantial reduction of LTP but appear behaviorally normal. Thus, CA1 LTP may need to be reduced below a certain threshold before behavioral defects become apparent.