Enriching the environment of young pigs reduces aggressive behaviour and increases exploratory behaviour. This difference in behaviour is maintained throughout life, even in the absence of enriching stimuli (Beattie et al., 1995). Although the exact neurochemical and neurophysiological mechanisms underlying aggression are not well understood there is evidence which indicates that low brain 5-hydroxytryptamine (5-HT) may be involved in the etiology of aggression (Burrows, 1999). This study examined the possibility that behavioural changes induced by environmental enrichment may be associated with subtle alterations in brain 5-HT chemistry.