SummarySuckling mice infected with reovirus type 3 were examined for changes in the epithelial brush border of the small intestine. After 3 days of infection with reovirus type 3, no significant changes were found in intestinal morphology or activity of any enzymes tested. After 6 days, villi were shortened and blunted with lymphangiectatic lesions and mild mononuclear infiltration in the lamina propria. In addition, there was a significant decrease in lactase (P< 0.001) and enterokinase activity (P< 0.05). However, there were no significant changes in the activities of alkaline phosphatase. In contrast, maltase (P< 0.001) and leucine amino-peptidase (P< 0.05) activities in the infected mice were significantly increased. These data suggest that brush border enzymes are affected differently by reovirus infection.SpeculationThe decrease in lactase activity and increase in maltase and leucine aminopeptidase in infant mice infected with reovirus type 3 can be a reflection of accelerated maturation subsequent to an increased turnover of the epithelial cells of the crypt and villi or to an induction of specific enzymes. In addition, it is possible that the severe decrease of lactase activity in comparison to other brush border enzyme changes is due to the fact that lactase is more vulnerable than are other enzymes to mucosal injury and the possibility that lactase is a receptor for the virus.