Rats with large thalamic lesions affecting the mediodorsal (MDn) and intralaminar (ILn) nuclei are impaired performing delayed matching to sample tasks (DMTS). To determine the neurological basis of this deficit, we trained rats to perform a place DMTS task and then compared the effects of excitotoxic lesions of the MDn, the ILn, and the lateral internal medullary lamina (L-IML). The MDn lesion had little effect. The ILn group was significantly impaired throughout 8 months of training. The L-IML group exhibited an intermediate level of impairment. Varying the sample response requirement, retention intervals, and trial-to-trial congruence in the side reinforced, had predicted effects, as did variations in response latency. However, none of these factors interacted with the treatment effects. These findings indicate that DMTS performance is disrupted by lesions of the ILn but not the MDn.