The behavior of rats with lesions of the fornix was compared with that of operated controls during the acquisition and extinction of an operant lever press response. Lesioned rats exhibited a small but significant increase in resistance to extinction of the lever press response. Detailed descriptive analysis of the behavioral reactions to the transition from acquisition to extinction revealed many clear-cut differences in behavior between lesioned and control rats. Controls reacted to the transition from acquisition to extinction with changes in frequency of bouts of behavior, in bout duration, in the sequential organization of behaviors, and in the emergence of new behaviors labeled as emotional. Many of these behavioral changes were either absent or significantly smaller in rats with fornix lesions. The utility of detailed descriptive analyses of behavior in differentiating between current theories of hippocampal function was discussed.