Eighty−four calves with diarrhoea were treated with fluids and 13 apparently healthy calves of similar ages were sampled as controls. Their total blood carbon dioxide (TCO2) was measured with a Harleco apparatus and 31 of the calves were treated with oral fluids and 53 with parenteral fluids. The oral fluid contained 1 18 mmol/litre Na+, 25 mmol/litre K+, 1 10 mmol/litre glucose, 108 mmol/litre bicarbonate (HCO3− as citrate), 43 mmol/litre Cl−, 4 mmol/litre Ca++, 4 mmol/litre Mg++ and 20 mmol/litre glycine, and the parenteral fluid contained 144 mmol/litre Na+, 4 mmol/litre K+, 35 mmol/litre HCO3− and 1 13 mmol/litre Cl−. Both treatments resulted in significant improvements in acid−base status as demonstrated by an increase in TCO2, and the treatment was successful in 27 of the 31 calves receiving oral fluids and in 45 of the 53 calves receiving parenteral fluids. Thirty−seven of the calves treated parenterally were very severely acidotic (TCO2 <8 mmol/litre) initially and they received an additional 400 mmol HCO3− added to the first 5 litres of infusion. Treatment was successful in 33 of these calves. The decision to administer additional bicarbonate was made on the basis of their acid−base status as measured with a Harleco apparatus. The strong ion difference (Na++K+−CI−) (SID) of the calves was calculated retrospectively. There was a significant correlation between the SID and TCO2 of the calves treated with oral fluids but not among the control calves or the calves treated parenterally. Furthermore, measurements of the change in SID during therapy gave little indication of the change in acid−base status as measured by the Harleco apparatus, with the SID decreasing (suggesting a worsening of acid−base status) in 16 calves in which the TCO2 increased (suggesting an improvement in acidbase status). There was a significant correlation between the change in SID and the change in TCO2 during treatment in the calves receiving oral fluids but not in the calves treated parenterally.