Micro-injections of the neurotoxin 5,7-dihydroxytryptamine into the dorsal raphe nucleus produced a behavioural profile in the social interaction test of anxiety similar to that seen in rats treated chronically with benzodiazepines. Neurotoxin injections into the median raphé nucleus did not produce a profile significantly different from that of the controls. In the control rats and in the rats with lesions of the median raphé nucleus, ACTH1-24 (corticotrophin) significantly reduced active social interactions, whereas it was without effect on the rats with lesions of the dorsal raphé nucleus. In the home-cage intruder test, the median raphé-lesioned rats submitted less to the intruder and stood and jumped on him more often than did the controls. The dorsal raphé-lesioned rats showed significantly fewer interactions of all kinds, compared with control rats when an intruder was placed in their home cages.