Rats with medial prefrontal cortical lesions were tested in a modified water maze navigation task. In Stage 1, the rats were trained to locate a hidden platform from a single start location. They were then subjected to a series of trials during which a second start position was used (Stage 2). In Stage 3, the rats had to navigate to a new goal location from the 2 experienced start positions. Stage 4 required the rats to navigate to the same goal as in Stage 3, starting from 4 distinct positions. Finally, a single probe trial with no platform was conducted. Rats with prefrontal lesions were impaired only during Stage 4. This deficit was specific to the 2 start positions newly introduced during this stage, suggesting a dysfunction of planning processes. This impairment might result from a working memory deficit, precluding the animal from forming an adequate representation of the whole course of movements required to reach the platform.