Rats with extensive lesions of the mamillary bodies and the immediately adjoining areas exhibited a severe impairment on two spatial memory tasks. In the first task, the animals had to retrieve food from each one of eight separate locations within the daily test session. In the second task, the animals were allowed to visit a particular location and retrieve food from it and, after a variable delay, they had to select between returning to the location they had already visited and a new location that now contained the food. Rats with lesions of the mamillary region were able to perform the task with minimal delay, but their performance was impaired as the delay increased. Finally, rats with mamillary lesions were not impaired in learning a visual discrimination task or its reversal. The results show that extensive damage in the mamillary region can lead to significant and long lasting spatial memory impairments.