Rats bilaterally implanted with cannulae in the CA1 region of the dorsal hippocampus and/or in the amygdaloid nucleus, in the entorhinal cortex, and in the posterior parietal cortex, were trained in a step-down inhibitory avoidance task. At various times after training (immediately, 30, 60 or 90min) they received, through the cannulae, 0.5µl microinfusions of saline or of 5.0µg of AP5 dissolved in saline. A retention test was carried out 24h after training. Retention test performance was hindered by AP5 given into hippocampus, amygdala, or both hippocampus and amygdala immediately but not 30min post-training. The drug was amnestic when given into the entorhinal cortex 30, 60 or 90min after training, or into the parietal cortex 60 or 90min after training, but not at earlier times. The findings suggest a sequential entry in operation, in the post-training period, of NMDA-receptor mediated mechanisms involved in memory processing; first in hippocampus and amygdala, 30min later in entorhinal cortex, and 30min later in posterior parietal cortex.