Proliferation of ETEC in the small intestine is recognised as the predominant cause of post-weaning colibacillosis (Marquardt et al., 1999). Established ETEC secrete toxins that disrupt normal enterocyte function and lead to diarrhoea, dehydration, poor performance and increased mortality. Zinc oxide (ZnO) addition to the diet increases small intestine mucosal growth and promotes normal intestinal function. In turn the incidence and/or severity of diarrhoea is reduced and performance is improved (Li et al., 2001, Ragland et al., 2006). Pre-weaning environment also influences performance with outdoor reared pigs growing faster to weaning than indoor reared contemporaries (Miller et al., 2007) but are outdoor reared pigs able to cope with ETEC challenge at weaning? The current experiment compared the responses of group-housed pigs weaned from different lactation environments and fed diets differing in ZnO content to a prescribed ETEC challenge.