Abnormal behaviour commonly develops in intensively managed horses. A possible cause is the change in diet occurring when the horse is stabled. An experiment was performed to examine this possibility by manipulating the diet with the feed supplement virginiamycin, as Founderguard. During 4 weeks, 18 horses were fed diets ranging from hay alone to concentrate plus hay in the ratio of 3:1. The rations of half the horses given concentrate were supplemented with Founderguard. Horses eating high concentrate rations displayed abnormal oral behaviours at a higher frequency than those eating only hay. The incidence of these behaviours was reduced when diets were supplemented with Founderguard. The drop in faecal pH of animals on concentrate diets was also reduced by Founderguard. Animals on concentrate diets had an average of 21 kg less gut fill post mortem. Dietary supplementation with virginiamycin as Founderguard apparently lessens some behavioural problems associated with management of stabled horses and the intake of grain. It may allow concentrate to be fed at higher levels than customary without adverse behavioural side effects. The suggested mechanism for the improved behaviour due to Founderguard supplementation is reduced fermentative acidosis in the hindgut.