Ion transport in the jejunal mucosa of 14-to 16-day-old piglets with severe diarrhea 40 hr after infection with transmissible gastroenteritis (TGE) virus was studied. In infected pigs Na+ transport failed to respond normally to glucose when studied either in Ussing short-circuited chambers or in suspensions of enterocytes isolated selectively from jejunal villi. Theophylline, 10mM, added to the chambers produced the same brisk electrical responses and increments in net Cl- secretion in tissue from both infected and control groups. A defect in glucose-stimulated Na+ absorption in the acute stage of a viral enteritis has been identified which probably contributes to the impaired lumen-to-extracellular fluid flux of Na+ found previously in the jejunum of intact TGE-infected pigs. The mechanisms causing diarrhea in this invasive viral enteritis differ from those causing toxigenic diarrhea.