Four lactating Friesian cows (average weight 485 kg, milk yield 22 kg d-1) were maintained in completely controlled circumstances and deprived of water for 72 hours. During this period they were carefully monitored and lost 100 kg in bodyweight, principally accounted for by cumulative losses of water in milk, urine, faeces and respired air. The mean rates of respiration and rumen contraction decreased by approximately 50 per cent. Mean body temperature increased by 0.5 degrees C, but pulse rate did not change significantly. Dry matter intake, particularly of hay, decreased rapidly to less than 10 per cent of normal on the third day. Milk yield decreased only slightly during the first 24 hours but on the third day the average yield was only 28 per cent of normal; the composition of the milk did not change significantly. There were significant progressive increases in serum sodium concentration (after four hours water deprivation), osmolality (after 24 hours), urea (after 38 hours), copper (after 48 hours) and magnesium and total protein concentration (after 62 hours); packed cell volume (measured with a Coulter Counter) increased after 38 hours but packed cell volume (determined in a microhaematocrit centrifuge) increased only after 62 hours. In spite of the dehydration the cows showed no signs of distress. Within 48 hours of the cows being given free access to water, bodyweight, appetite, milk yield and blood composition had returned almost completely to normal.