Foals at pasture with their dams were studied during the preweaning and postweaning periods to determine whether their behaviour was associated with their gender, mare rank, and the development of abnormal oral behaviour. The behaviour of males and females hardly differed, but mare rank affected social interaction patterns and suckling behaviour of foals, with foals of subordinate mares involved in more subordinate behavioural interactions. Foals of subordinate mares spent more time in perisuckling activities such as teat nuzzling than foals of higher ranking mares. Eighteen foals (out of 186) developed abnormal oral behaviour before weaning, and 42 developed such behaviour after weaning. The development of abnormal oral behaviour in the preweaning phase was associated with suckling behaviour. Foals exhibiting abnormal behaviour during the preweaning phase were involved in more suckling terminations within suckling bouts and pushed the udder with their nose most often compared with normal foals or foals that developed abnormal oral behaviour in the postweaning phase. Foals that did not exhibit abnormal oral behaviour in the preweaning period, but exhibited the behaviour in the postweaning period, spent more time suckling and twice as much time teat nuzzling as other foals. The results of this study provide further evidence that there is an association between digestive function and abnormal oral behaviour in horses.