Adult male rats were fed a low or high fat diet and given psychosocial stress (crowded and unstable housing with daily predator exposure) for 3 weeks. Neither stress nor high fat diet, alone, produced dendritic atrophy; only the group given the combination of stress and high fat diet developed a reduction of the length and number of branch points of apical dendrites of CA3 neurons. These findings indicate that a synergy between high fat diet and stress caused a retraction of CA3 dendrites. The findings are consistent with work on peripheral (e.g., cardiovascular) systems demonstrating a synergy between stress and high fat diet, and are relevant toward understanding how diet and stress interact to adversely affect brain and memory processing.