Current views of delayed matching-to-sample (DMTS) performance in pigeons postulate the active maintenance (rehearsal) of a representation of the sample stimulus during the delay in­ terval. Honig and Wasserman (1981) termed this type of processing retrospective. In an alter­ native account, prospective processing, a representation of an upcoming comparison (test) stim­ ulus is maintained during the delay interval, rather than a representation of the sample. The present experiments attempted to determine whether processing is retrospective or prospective in DMTS, as well as in delayed simple discrimination (a related procedure). The results of the experiments indicated that although pigeons are capable of retrospective processing, prospec­ tive processing appears to be the process typically used when there are no constraints that would hinder or prevent its use. This finding is contrary to the usual assumption of retro­ spective processing. Short-term or active memory in laboratory animals has been a popular topic in recent years, with the development of memory tasks such as delayed matching to sample (DMTS). Much of the recent research investigating how animals are able to per­ form tasks such as DMTS has favored the interpre­ tation of an active process (rehearsal) of some coded form of an initial sample stimulus (which is no longer present) to bridge the delay between sample and com­ parison stimuli (Grant, 1981; Maki, 1981; Maki &