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The orbital prefrontal cortex (OPFC) might be particularly involved in adapting instrumental responses to changes of stimulus–reward contingencies. We investigated whether signals in the OPFC transmitted via N-methyl-D-aspartate (NMDA) receptors are critical for learning a reversal of stimulus–reward contingencies. Rats were trained in a reaction time (RT) task demanding conditioned lever release with discriminative stimuli signalling in advance the upcoming reward magnitude (one or five pellets). After acquisition, RT of responses with expectancy of high reward magnitude were significantly shorter. Thereafter, stimulus–reward contingencies were reversed and rats received intra-OPFC infusions of saline or of the NMDA receptor antagonist AP5. Reversal learning was tested for 6 days, with microinfusions being given on days 1, 3 and 5. Intra-OPFC blockade of NMDA receptors impaired the learning of a reversal of previously acquired stimulus–reward magnitude contingencies: (i) latencies of correct responses were generally shortened, regardless of the response-associated reward magnitude; (ii) the proportion of premature responses was increased; and (iii) responses were not guided by the current significance of the reward-predicting stimuli. These findings provide novel evidence for NMDA-receptor-dependent plasticity in the OPFC in reversal learning.