Separate groups of rats with lesions in the parietal cortex (PC) or hippocampal formation (HF) were tested for acquisition and retention of the Morris water maze cognitive mapping task. Some of the animals in each lesion group received preoperative training in the task. Other animals in each group received no preoperative training. The results indicate that although both lesions lead to a cognitive mapping impairment in both the acquisition and retention of the task, the animals with PC lesions were more severely impaired than were the animals with HF lesions, as indicated by quantitative measures. However, qualitative aspects of the animals' swim behavior indicate that the HF damaged animals tend to use "nonmapping" strategies to solve the tasks, which suggests that the qualitative nature of their impairment differs from that of the PC damaged animals. The results of this study support the hypothesis that PC plays an important role in the processing of information about space that is allocentric or external to the body.