The present experiment evaluated the ability of rats with lesions in the thalamic mediodorsal nucleus (MD) to perform a task which differentiates between "reference" and "working" memory. In this task, only four of the arms of an 8-arm radial maze were baited, and animals were to restrict their entries to arms which were baited and to avoid never-baited arms. Despite several postoperative acquisition trials, rats with MD lesions did not acquire the task to a degree comparable to control subjects. Subjects with lesions continued to enter never-baited arms (reference memory errors) and to reenter baited arms (working memory errors). Given the lack of specificity in the behavioural impairment, the reference-working memory distinction seems to be an inappropriate one for characterizing the MD lesion deficit in rats. This deficit may involve an inability to use environmental stimuli to distinguish among arms of the maze, or an alteration in motor mechanisms.