The permeability properties of the small intestinal mucosa was investigated in nine previously healthy children with acute diarrhoea due to rotavirus. The investigation was performed after intake of a mixture in water of polyethyleneglycol molecules (PEG 400 and 1000) ranging from 282 to 1250 dalton in molecular weight. The 6-h urinary recovery of the PEGs was determined with high performance liquid chromatography and used to assess the permeability characteristics of the intestine. The patients served as their own controls and were investigated in the same manner after recovery 3-5 weeks later. A significantly lower urinary recovery of PEG was noted for all molecular sizes (326-1206 dalton) during acute diarrhoea in comparison with the results obtained after recovery (p less than 0.001-0.1). There was also a relatively lesser change in the urinary recovery of larger PEG molecules during infection, as reflected by a higher recovery ratio between 1074 and 370 dalton PEGs. The results indicate profound changes in the permeability characteristics of the intestine during acute rotavirus diarrhoea.