Separate age groups of C57BL/6 and autoimmune New Zealand Black (NZB) mice were compared for diazepam-induced ataxia and barbiturate-induced loss of righting reflex. Between 1 and 3 months of age, both strains showed a similar age-related decrease in ED50 for diazepam-induced ataxia. However, between 3 and 12 months the decrease in ED50 was markedly greater in NZB mice. In contrast, age-related increases in the durations of loss of righting reflex following hexobarbital or barbital were similar in both strains. The results suggest that NZB mice show relatively accelerated age-related increases in sensitivity to benzodiazepine, but not to barbiturates.