Background & aims: Identification of the mTOR pathway as an amino acid sensing mechanism which regulates protein synthesis led us to investigate its role in rotavirus diarrhea. We hypothesized that malnutrition would reduce jejunal protein synthesis and mTOR signaling via its target, ribosomal p70 s6 kinase (p70^s6k). Methods: Newborn piglets were artificially fed from birth and infected with porcine rotavirus on d 5 of life. Study groups included infected (fully-fed and 50% protein-calorie malnourished) and non-infected fully-fed controls. Initially, in "worst-case scenario" studies, malnourished infected piglets were euthanized on d 1, 3, 5, and 11 post-inoculation, and jejunal samples were compared to controls to determine the time-course of p70^s6k activation. Using a 2x2 factorial design, we subsequently determined if infection and/or malnutrition affected mTOR activation on d3. Western blotting and immunohistochemistry were used to measure total and phosphorylated p70^s6k; ³H-phenylalanine incorporation was used to measure protein synthesis; and lactase specific activity and villus-crypt dimensions were used to quantify injury. Results: At the peak of diarrhea, in vitro jejunal protein synthetic rate increased 2-fold (compared with uninfected jejunum), concomitant with increased jejunal p70^s6k phosphorylation (4-fold) and increased p70^s6k level (3-fold, p<0.05). Malnutrition did not alter the magnitude of p70^s6k activation. Immunolocalization revealed that infection produced a major induction of cytoplasmic p70^s6k and nuclear phospho-p70^s6k, mainly in the crypt. A down-regulation of semitendinosus muscle p70^s6k phosphorylation was seen at d1-3 post-inoculation. Conclusions: Intestinal signaling by p70^s6k was not inhibited by malnutrition, but was strongly activated during an active state of mucosal regeneration.