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Assessed the role of N-methyl-{d}-aspartate (NMDA) receptors in the amygdala in associative fear conditioning. 132 rats were implanted with cannulae aimed at the basolateral amygdaloid nuclei, while 13 controls had cannulae aimed at the interpositus nuclei of the cerebellum. NMDA antagonists infused into the amygdala blocked the acquisition, but not the expression, of fear conditioning measured with a behavioral assay mediated by a defined neural circuit (fear-potentiation of the acoustic startle reflex). This effect showed anatomical and pharmacological specificity, and was not attributable to reduced salience of the stimuli of light or shock used in training. An NMDA-dependent process in the amygdala was critical for associative fear conditioning.