Suckling calves suffering from diarrhea at various ages were examined. Striking differences in the biochemical disturbances were observed in plasma between postnatal (2.3 days old, on average) and older calves (14.7 days old). Severe dehydration and hypovolaemia in the youngest calves was accompanied by a marked acidosis with hyperlactataemia, hyperalaninaemia and a considerable increase of most other amino acids. Simultaneously, there was an increase of plasma potassium and of plasma magnesium. In contrast, in diarrheic calves more than 6 days old, acidosis was also observed with minor changes in plasma amino acids and without hyperlactataemia. There were only minimal changes in the plasma magnesium concentration in the older calves. No evident lipomobilization was observed during the course of the disease.