The purpose of this study was to measure the net absorption of fluid, sodium, potassium and chloride in the small intestine of weaned pigs and of their unweaned littermates and to correlate these values with villus height and crypt depth. Five pairs of segments of the small intestine were prepared in each of 80 pigs; the cranial segment of each pair was injected with an enterotoxigenic strain of Escherichia coli and the caudal segment with a control solution. Net absorption was measured on the day of weaning and four, seven, 11 and 14 days after weaning. In unweaned pigs the net absorption of fluid, potassium and chloride did not vary with time. In weaned pigs the net absorption of fluid in the control segments was significantly less on days 4, 7 and 14 after weaning and of sodium and chloride on days 4 and 7 than in unweaned littermates. In infected segments of weaned pigs the net absorption of fluid was significantly less than in unweaned littermates on day 11 and 14, of sodium and potassium on day 11 and of chloride on days 4 and 11 after weaning. Net absorption was negatively correlated with villus height but only in the infected segments of weaned pigs; no other correlations were found. It was concluded that after weaning the net absorption of fluid and electrolytes in the small intestine of pigs is temporarily decreased, a condition that may initiate diarrhoea.