Male white Swiss mice that had previously been made aggressive by several weeks of individual housing were allowed to fight for 5 to 10 minutes each day for 5, 10, or 14 consecutive days; fighting caused a marked enlargement of their adrenals, spleens, and hearts, and a large increase in adrenal catecholamines; brain catecholamines were slightly increased. Long-term group caging, under conditions where the mice did not fight, caused changes that were directionally the same but of smaller magnitude. Similar sociophysiological influences may be important in natural populations. Fighting mice, used under well-defined and closely controlled conditions, may be useful for studying normal mechanisms of neuroendocrine adaptation and control, and, possibly, for studying some forms of hypertension and cardiovascular-renal disease.