Behavioural Brain Research, 2 ( 1981 ) 211-218 211 © Elsevier/North-Holland Biomedical Press EFFECTS OF ISOLATION ON ACTIVITY, REACTIVITY, EXCIT- ABILITY AND AGGRESSIVE BEHAVIOR IN TWO INBRED STRAINS OF MICE BERT SIEGFRIED, ENRICO ALLEVA, ALBERTO OLIVERIO and STEFANO PUGLISI- ALLEGRA* lstituto di Psicobiologia e Psicofarmacologia, C.N.R. and lstituto di Fisiologia Generale dell'Univer- sire, Rome (Italy) (Received June 25th, 1980) (Accepted July 1st, 1980) Key words: isolation - activity - reactivity - excitability - aggression - C57BL/6 mouse - DBA/2 mouse SUMMARY In order to investigate mechanisms of isolation-induced aggressive behav- ior, inbred mice of the C57BL/6 and DBA/2 strains were individually housed over a period of 8 weeks. In the DBA/2 strain only, isolation was followed by a clear increase in activity (Animex), reactivity (reactions upon tactile body stimulation), excitability (duration of EEG desynchronization elicited by tactile stimulation of the thorax area under urethane anesthesia) and intermale aggres- sion (biting and fighting responses). The use of inbred strains of mice proved to be a useful tool for the examination of the relationship between various parameters. It is concluded that there are no clear correlations between activity, reactivity and aggressive behavior and that the resulting aggressive responses in the DBA/2 strain are likely due to the increase of excitability. INTRODUCTION It is well known that a prolonged period of individual housing may induce aggressive behavior in mice [15], through mechanisms which were recently * To whom requests for reprints should be addressed at: Istituto di Psicobiologia e Psico- farmacologia, C.N.R., Via Reno 1, 00198 Rome, Italy.