Adult, male albino mice were isolated for 17 days. A category of 20 animals were injected intramuscularily, on alternate days, with 40 µg ACTH 4–10 peptide, which has been shown to be a potent fraction in modifying fear-mediated behaviours in the rat; 20 other animals received a placebo. On the 18th day of isolation, each mouse was given a 5 min aggression test against a ‘standard opponent’ and, on the 19th day of treatment, peptide- and placebo-treated animals were paired in the conflict situation. No effects of the peptide on isolation-induced aggressive behaviour could be demonstrated but, unlike in the rat, the peptide-treated animals showed increased adrenal weights. In view of the demonstrated suppressant action of the entire ACTH molecule on this behaviour, the possible explanations for this result are that ACTH exerts its action by stimulating glucocorticoids (the small ACTH fraction is said not to mediate steroidogenesis, but the adrenal weight increase argues against this) or that another part(s) of the ACTH molecule is involved in the neural action on agonistic behaviour.