Twelve diarrhoeic calves were treated intravenously with an isotonic solution containing sodium bicarbonate, and their oxygen equilibrium curves (OECs) were calculated under standard conditions and compared with those of a group of healthy calves. The relationships between the OECs for arterial and venous blood and the oxygen extraction ratio were investigated. In the diarrhoeic calves, the affinity of haemoglobin for oxygen, measured under standard conditions, was increased compared with the healthy animals. During the infusion, the standard partial oxygen pressure at 50 per cent saturation of haemoglobin (P50) values stayed below the values recorded in the healthy animals. At the end of the infusion the mean standard P50 of the diarrhoeic calves was lower than before the infusion. The combined effects of all the regulating factors on blood oxygen binding resulted in the OECs of the arterial and jugular venous blood of the diarrhoeic calves remaining unchanged compared with the healthy calves. However, the administration of the infusion decreased the P50 of both the arterial and venous blood to below the value recorded in the healthy calves. Oxygen extraction by the tissues was impaired in the diarrhoeic calves throughout the infusion, and they remained dehydrated and depressed until 120 minutes after the infusion began.