Ten calves were challenged with one of two strains of reo-like virus (rotavirus). Changes in the daily faecal and urinary outputs were monitored and packed cell volume, plasma sodium, potassium and urea levels were measured. Faeces were examined for the presence of rotavirus by direct electron microscopy and immunofluorescence in cultures of PK(15) cells. All calves excreted rotavirus in the faeces for several days. Two calves remained clinically normal throughout the experiment, but in the remaining calves, faeces became mucoid in consistency and yellow-white in colour. In only two calves did the daily faecal output exceed 500 g with a fall in the dry matter content to less than 10 per cent. Slightly elevated blood urea levels and hyperkalaemia were the only changes observed in blood chemistry and these quickly returned to normal. Virus antigen was observed in the epithelial cells by immunofluorescence in the proximal and middle small intestine of calves. Pathological lesions occurred predominantly in the proximal small intestine of nine calves examined.