A reduction in alpha5 subunit-containing gamma-aminobutyric acid (GABA)A receptors has been reported to enhance some forms of learning in mutant mouse models. This effect has been attributed to impaired alpha5 GABAA receptor-mediated inhibitory modulation in the hippocampus. The introduction of a point mutation (H105R) in the alpha5 subunit is associated with a specific reduction of alpha5 subunit-containing GABAA receptors in the hippocampus. The present study examined the modulation of associative learning and the extinction of conditioned response in these animals. The strength of classical conditioning can be weakened when a trace interval is interposed between the conditioned stimulus and unconditioned stimulus. Here we report that this 'trace effect' in classical conditioning was absent in the mutant mice--they were insensitive to the imposition of a 20-s trace interval. This effect of the mutation was most clearly in the female mice using an aversive conditioning paradigm, and in the male mice using an appetitive conditioning paradigm. These gender-specific phenotypes were accompanied by a resistance to extinction of conditioned fear response in the mutant mice that was apparent in both genders. Our results identify neuronal inhibition in the hippocampus mediated via alpha5 GABAA receptors as a critical control element in the regulation of the acquisition and expression of associative memory.