Intestinal absorption of ovalbumin (OVA), a dietary macromolecule, was studied in malnourished and normally nourished suckling mice after experimentally induced infection with rotavirus. All mice developed diarrhea within 24 to 48 h postinoculation. The malnourished animals exhibited more severe symptoms and an increased number of rotavirus-containing enterocytes in intestinal sections as compared to well-nourished mice when examined 3 d postinoculation, at the peak of diarrhea. Histopathologic examination revealed villus atrophy and pronounced vacuolization of villus enterocytes in association with malnutrition and rotavirus infection. The combination of malnutrition and viral infection resulted in more severe mucosal damage, including disruption of microvillus borders. After a single oral dose of 100 micrograms OVA at 3 d postinoculation, the concentration of OVA in serum, gastric content, intestinal lavage fluid, and intestinal tissue homogenates was measured at different time intervals. The concentrations of OVA in intestinal tissue were significantly higher in malnourished animals, whereas lower values were found in rotavirus-infected animals. In all mice, OVA was rapidly absorbed and could be consistently detected in the serum within 5 min. OVA levels peaked at 45 to 60 min and then gradually declined. In malnourished infected animals, the uptake of OVA was rapid and resulted in significantly higher serum levels when compared to well nourished or uninfected controls, respectively. The peak uptake of OVA per g body wt was about 4.5 times more in malnourished infected compared to well-nourished infected mice and 2.5 times higher in normally nourished infected animals when compared to uninfected controls. These results indicate that rotavirus infection in association with malnutrition may cause a significant rise in gut permeability to environmental macromolecules.