The effects of bilateral electrolytic lesions of the 'shell' subterritory of the nucleus accumbens in the rat were examined on 2 tasks known to be sensitive to hippocampal damage. Experiment 1 tested the effects of shell lesion on delayed non-matching-to-sample (DNMS) task in a T-maze. The maze was rotated 180°after the end of acquisition. Experiment 2 used a 4-arm baited, 4-arm unbaited. 8-arm radial-maze task and its reversal. Shell lesion led to impaired acquisition of DNMS in a T-maze and of 4-arm baited, 4-arm unbaited, 8-arm radial maze tasks, suggestive of mnemonic deficits. Following analysis of animals' choice pattern in both tasks, the deficit was interpreted as being largely due to an extensive use of response strategy. The result suggest that the inappropriate use of response strategy by shell animals was a result of their inability to switch from initial response strategy to a later, more appropriate, memory-dependent strategy.