Mucosal Na, K-ATPase activity was studied in rectal biopsy specimens from 19 children with ulcerative colitis (UC) (mean age, 13 years) and 4 children with Crohn's colitis (mean age, 14 years) and compared with biopsy specimens from 12 control children (mean age, 12 years). The Na, K-ATPase activity was significantly decreased in UC with severe rectal inflammation compared with UC in remission or with children with unspecific symptoms and normal mucosa (p < 0.001, respectively). A higher enzyme activity was shown with age in the group with normal rectal mucosa and no evidence of inflammatory bowel disease (n = 17). The decreased Na, K-ATPase in UC with severe rectal inflammation might contribute to the diarrhoea by impairment of sodium transport.