AbstractForty‐six female mice (CFW) were isolated for a period of 23 weeks. The effect of isolation on fighting behavior was tested weekly by introducing a naive brown female mouse into the subject's home cage. Total leucocyte counts were obtained at 8 and 14 weeks of isolation. The appearance of leucopenia was used as an index of elevated adrenocortical activity. After 23 weeks of isolation all animals were sacrificed by decapitation. Plasma was collected for corticosterone assay, and paired adrenals were used to assay catecholamine levels. On the basis of the frequency and/or the absence of fighting, the mice were segregated into fighters (n = 22) and non‐fighters (n = 17). Analysis of the data by Pearson's product moment correlation and Student's t‐test showed that elevated sympathetic‐adrenal activity was positively correlated with aggression and that elevated adrenocortical activity was negatively correlated with agg