Uncontrolled inflammatory responses in the immature gut may play a role in the pathogenesis of many intestinal inflammatory syndromes that present in newborns or children, such as necrotizing enterocolitis (NEC), idiopathic inflammatory bowel diseases (IBD), or infectious enteritis. Consistent with previous reports that murine intestinal function matures over the first 3 weeks of life, we show that inflammatory signaling in the neonatal mouse gut increases during postnatal maturation, with peak responses occurring at 2-3 weeks. Probiotic bacteria can block inflammatory responses in cultured epithelia by inducing the generation of reactive oxygen species (ROS), which inhibit NF-kappaB activation through oxidative inactivation of the key regulatory enzyme Ubc12. We now report for the first time that the probiotic Lactobacillus rhamnosus GG (LGG) can induce ROS generation in intestinal epithelia in vitro and in vivo. Intestines from immature mice gavage fed LGG exhibited increased GSH oxidation and cullin-1 deneddylation, reflecting local ROS generation and its resultant Ubc12 inactivation, respectively. Furthermore, prefeeding LGG prevented TNF-alpha-induced intestinal NF-kappaB activation. These studies indicate that LGG can reduce inflammatory signaling in immature intestines by inducing local ROS generation and may be a mechanism by which probiotic bacteria can prevent NEC in premature infants or reduce the severity of IBD in children.