Separate groups of rats of three ages (6 month, 15 month or 24 month) were trained in a two-lever operant chamber on one of two versions of a paired-trial delayed response task involving either matching or non-matching of the choice response to a sample lever. The older rats were unimpaired in learning either version of the task during initial training with no (0 s) delay between the sample and choice responses. However, when variable 0-24 s delay intervals were introduced, the 24-month group was impaired on acquisition of the delayed non-matching task, and both the 15- and 24-month groups were impaired on acquisition of the delayed matching task compared to the 6-month group. Deficits in the older groups in asymptotic performance were attributable to an impairment at longer delay intervals whilst maintaining near perfect performance at the shorter delay intervals, suggesting a selective short-term memory impairment. The delay-dependent deficits of the older groups were not ameliorated by the muscarinic agonist arecoline or the cholinesterase inhibitor physostigmine, and so failed to corroborate a cholinergic interpretation of the observed age-related impairment in short-term memory.