Benzodiazepine and anticholinergic drugs interfere with septo-hippocampal function in similar but not identical ways. They also share a number of common behavioural effects and, in particular, both classes of drug interfere with spatial memory in the Morris Water Maze--a test which is very sensitive to hippocampal dysfunction. We have previously shown that the anticholinergic drug scopolamine impairs discriminability, but not rate of forgetting, in delayed conditional discrimination. In the present study forgetting was quantified by fitting a negative exponential function to estimates of discriminability derived from a signal detection analysis of data from an auditory delayed conditional discrimination task. Chlordiazepoxide produced a highly significant decrease in discriminability which was monotonically related to the logarithm of dose in the range 0.67-18.0 mg/kg IP. The rate of forgetting was not increased. These data confirm the pharmacological independence of changes in discriminability and rate of forgetting; demonstrate that in this task chlordiazepoxide has similar effects to scopolamine; and suggest that the effects of chlordiazepoxide in other working memory tasks could be more a result of changed stimulus processing than impairment of memorial processes.