This study was designed to investigate the potential effects of medial mammillary nucleus (MMn) lesions on spontaneous alternation behavior in rats. Behavioral measurements were made in a water T maze in which the side arms were not differentially reinforced. Spontaneous alternation was tested before and after surgery alone. In both experiments, volumetric estimates of the MMn lesion were made by stereologic calculations. The results obtained do not support the direct participation of the MMn in retrograde or anterograde memory processes related to spontaneous spatial alternation.