The role of the mammillary bodies in spatial function was investigated by testing the ability of rats to perform a single-alternation habit. Mammillary-damaged and control animals were tested under conditions which provided for a brief delay or for no delay between successive trials. Experiment 1 tested animals on acquisition of the habit; the results indicate that mammilary lesion impairs learning of the spatial alternation habit and that intertrial delay adds significantly to the dysfunction. The results of experiment 2 suggest that retention of the habit is not impaired when the lesion is inflicted after the initial learning. Both experiments suggest that the mammillary bodies are essential in the storage and/or retrieval of recent maze choices and support the position that they play a role in spatial memory. The evidence also suggests that mammillary body deterioration is responsible for the spatial confusion which characterizes humans suffering from Korsakoff's psychosis.