We studied the influence of diet during diarrhea on gut mucosal barrier in a suckling rat model. Rat pups were inoculated with IDIR virus (a group B rotavirus) at 10 d of age. Beginning 2 d postinfection, in addition to maternal milk, group CM received a daily gavage of cow milk and group GG received Lactobacillus casei strain GG, a human strain previously shown to survive the passage through the gastrointestinal tract and temporarily colonize the gut. Group CMGG received a combination of these, and control animals were gavaged with tap water. At 21 d of age, jejunal absorption of intact and degraded horseradish peroxidase (HRP) in Ussing chamber was markedly higher in IDIR virus-infected than in noninfected controls. In the two groups gavaged with cow milk, group CM and group CMGG, the numbers of specific antibody-secreting cells (enumerated by the solid-phase enzyme-linked immunospot assay) against beta-lactoglobulin were significantly higher than in the groups that had not received cow milk. In parallel with immune system activation, a statistically significant increase in the absorption of intact HRP (mean and 95% confidence interval, ng x h-1 x cm-2) was detected: group CM, 302 (155, 586); group CMGG, 174 (56, 545); infected controls, 121 (57, 257); and group GG, 44 (8, 254). A decrease in the uptake of intact HRP (F = 3.64, p = 0.06) and degraded HRP (F = 9.50, p = 0.004) was associated with the introduction of L. casei GG to the diet, irrespective of coexposure to cow milk.(ABSTRACT TRUNCATED AT 250 WORDS)