In a first set of experiments the posttrial intraperitoneal acute administration of the GAD-inhibitor DL-allylglycine impaired the memory processes of C57BL/6 (C57) mice tested in a passive avoidance apparatus. Memory improvements were, on the contrary, observed following morphine treatment. These effects were dose- and time-dependent. They were, in particular, evident if mice were injected immediately, or 30 min, but not 60 min, after training in the apparatus, and were absent in the no-footshock groups, suggesting lack of proactive effects of the drugs on performance. In a second set of experiments, the posttraining administration of allylglycine, at a per se ineffective dose (5 mg/kg), antagonized the memory improvement exerted by the administration of morphine (0.5 or 1.0 mg/kg). The results support the hypothesis of an involvement of GABAergic mechanisms in the memory consolidation processes, and suggest the possibility that GABAergic systems play a role in the effects of morphine on memory in C57BL/6 mice.