The N-methyl-D-aspartate (NMDA) receptor plays an important role in acquisition of memory after one-trial passive avoidance training in day-old chicks. Here, we examined the effect of D-cycloserine, a partial agonist of the glycine site on the NMDA receptor, on memory retention after training on weak aversive stimuli (10% methyl anthranilate). Bilateral, or unilateral, intracranial injections of 5 microliters of 600 mumol.l-1 D-cycloserine (DCS) were made into the intermediate and medial hyperstriatum ventrale 5 min posttraining and chicks were tested either 30 min or 1, 6, or 24 h later. Enhanced memory retention (number of chicks avoiding) occurred after bilateral, or left hemisphere, injections of DCS (compared to saline-injected controls), when chicks were tested at 1 and 6 h, but not at 30 min or 24 h posttraining. These data suggest that activation of the glycine site on the NMDA receptor in the immediate posttraining period can transiently extend the retention period of memory for a weak aversive stimulus.