Rats were trained for 1,345 spatial nonmatching-to-sample (NMTS) trials, matched, assigned to pyrithiamine-induced thiamine deficiency (PTD) or control treatments, recovered, and re-tested for 400 trials of NMTS. The PTD model produced two bilaterally symmetrical lesions: one of medial thalamus that was centered on the internal medullary lamina (IML) and another involving the mammillary bodies. PTD rats with complete IML lesions showed a sharp drop in performance that persisted throughout posttreatment training. PTD rats with IML sparing were impaired immediately after treatment but improved to a level comparable to that of controls. For all animals, NMTS accuracy decreased for longer latency responses. PTD animals differed from controls primarily in the low frequency and inaccuracy of their short-latency (0-2.9 s) responses. The improvement of the PTD rats with IML sparing was marked by an increase in both the number and accuracy of short-latency responses.