We have developed a pig caecal cannulation model that allows us to evaluate the effects in vivo of feed withdrawal on (1) the caecal environment, including pH and volatile fatty acid (VFA) concentration, and (2) the growth of foodborne pathogens in the caecum. In vitro studies evaluated growth of Yersinia enterocolitica and Salmonella typhimurium at five concentrations of VFA at four pH levels. Minimal growth occurred in VFA and pH levels that simulated the caecum of a well-fed pig. Maximal occurs in the absence of VFA (0 mM/ml) at pH 7.0. When cultured in the caecal contents of a fasted pig, Yersinia and Salmonella replicate and survive. In contrast, caecal contents of a well-fed pig inhibit their growth in vitro. When instilled directly into the pig caecum, Y. enterocolitica and S. typhimurium were detected in fecal and cecal samples for up to 1 month. Infected pigs were subjected to four cycles of interrupted feeding. No predictable change occurs in the number of Yersinia or Salmonella in the caecum or in feces of pigs subjected to interrupted feedings compared with controls on a normal feeding regimen. In contrast, a fasting cycle predictably reduced VFA concentrations and increased the pH of the caecum. Thus, the pig caecal cannulation model is a practical way of monitoring the long-term dynamics of growth and survival of foodborne pathogens in the live animal.