SUMMARY: In an attempt to clarify the role of gonadal function in the release of anti-aggression pheromone into the urine of female mice, two experiments were carried out. Each involved recording the aggressiveness of 'fighter' male mice when presented with castrated male 'opponents' which had been treated either with urine from one of four categories of female donor mice, or with water. In the first series of experiments it was shown that the urine from oestrous and dioestrous mice contained similar amounts of anti-aggression activity. Both spaying and testosterone propionate (TP) injection of the female urine-donors abolished this activity. In a second series of experiments urine from spayed donors injected with TP greatly increased the aggression response to a level which exceeded that obtained with the urine of TP-treated intact mice. High doses of oestrogen resulted, in the spayed urine-donors, in an actual increase in aggression response. Both treatments increased clitoral gland weight, and it is concluded that androgen or high levels of oestrogen can stimulate the clitoral gland of the female, a possible consequence being a release of aggression-promoting pheromone. The aggression-inhibiting pheromone, which depends on the ovary for its release, is probably the product of some other tissue, as yet unidentified.