Three experiments compared the effects of lesioning areas of thalamus, cortex, and the hippocampal system on delayed matching (DMTS) and nonmatching (DNMTS) to sample. Temporal decay was measured by comparing performances at different retention intervals (RIs) for rats trained to stability. Lesions of the lateral-internal medullary lamina site in thalamus and the medial wall area in frontal cortex produced impairments that were significantly greater than for lesions of the mediodorsal nucleus in thalamus, the fornix, or the dorsal hippocampus. The effects of lesions on temporal decay differed depending on how RIs were manipulated. When RIs were manipulated within training sessions, the DMTS and DNMTS impairments were delay independent (i.e., none of the lesions increased the rate of temporal decay). When RIs were manipulated between sessions, thalamic lesions were associated with an increase in the rate of temporal decay of DNMTS.