In this study four goats were investigated during 48 h of water deprivation. Milk production was 3.1 plus minus 0.2 l during control days and dropped to 2.2 plus minus 0.2 l after 24 h, and to 1.2 plus minus 0.1 after 48 h of water deprivation. This corresponded to a 20 per cent fall in milk vein blood velocity (MBV) the first day and a 40 per cent decrease in MBV the second day. Plasma volume fell only from 2.8 plus minus 0.1 to 2.4 plus minus 0.3 l after 48 h water deprivation. This indicates a local regulation within the mammary gland. Plasma arginine vasopressin (AVP) increased from 1.3 plus minus 0.3 to 10.4 plus minus 1.7 pmol per l on the second day of water deprivation and it can not be excluded that AVP is involved in regulating the distribution of blood flow to the udder in the dehydrated goat. Total milk protein concentration fell during water deprivation which was shown to be caused by a decrease in the casein fraction.