It has been widely reported that the pigs responsible for tail biting under both commercial and experimental conditions, are those which show poor growth rates. This may be because of an inability to obtain food because of social factors or poor pen design, or an inability to utilise food because of health or metabolic disorders. A major reduction in protein:energy ratio in the diet has been shown to increase attraction to blood in an experimental tail chewing model (Fraser, 1987). The aim of this study was to clarify whether the increased attraction to blood is a consequence of a specific metabolic state resulting from a protein deficiency (with effects on neurotransmitter pathways, Harper and Peters, 1989), or whether it occurs under conditions of a reduced growth rate irrespective of a protein deficiency.