We have used an animal model of aggression, the isolation syndrome in mice, to examine the possible role of the trace amines, beta-phenylethylamine (PEA), meta-tyramine (m-TA) and para-tyramine (p-TA) in aggressive behaviour. The brain, plasma and urinary levels of PEA, m-TA and p-TA, and their respective major acid metabolites, phenylacetic acid (PAA), meta-hydroxyphenylacetic acid (m-HPA) and para-hydroxyphenylacetic acid (p-HPA) were measured in isolated aggressive mice (after fighting), and in group housed controls. The urinary levels of PEA, m-TA, PAA, m-HPA and p-HPA, and the plasma levels of PAA and p-HPA were significantly lower in isolated aggressive mice. Similarly, the whole brain levels of PEA, p-TA, PAA and p-HPA tended to be reduced. In contrast, the brain levels of m-TA and m-HPA tended to increase. It should be noted, however, that the present procedure did not dissociate the stress and aggression components of the isolation syndrome, and, therefore, further experiments are required to determine whether the observed neurochemical changes are functionally related to increased aggression.