Electrolytic lesions were placed in either the dorsal or median raphe nuclei of adult male albino rats. Both lesions produced significant reductions in forebrain serotonin levels. Lesions of the dorsal nucleus produced a long-lasting increase in pain-elicited aggression, whereas median lesions were without effect. By contrast, lesions of the median nucleus produced significant increases in open-field activity, which began immediately and lasted for at least 3 mo, whereas lesions of the dorsal nucleus had no such effect. Similarly, when animals with dorsal or sham lesions were tested in an open field and then given a brief noncontingent footshock, their open-field activity was markedly reduced on the following day. Median animals, however, showed little or no decrease in open-field activity on the day after footshock. These results suggest that the serotonin-containing neurons of the median raphe nucleus may exert an influence over the emotional responsivity of the rat. The overall results of these experiments extend our previous reports that lesions specific to the dorsal nucleus produce markedly different behavioral effects than lesions confined to the median nucleus. They also challenge the utility of manipulations that fail to take this into account, e.g., lesions of more than one nucleus or depletion of serotonin throughout the central nervous system.