The quantity of water in the body is a variable of critical importance for the successful maintenance of the individual. The total body water (TBW) content is normally kept within fairly narrow limits, the upper and lower levels of which are determined by the animal’s homeostatic control mechanisms. When TBW moves into relative deficit it is monitored by internal sensing systems which activate the behaviour concerned with drinking. As drinking proceeds, the deficit is progressively reduced or corrected so that eventually balance is restored and the drinking stimulus is removed. It has been shown that in the absence of food pigs drink insufficient water during the first 24 h period when water is reintroduced to repair fully the gross deficit caused by a period of deprivation. The physiological adjustments induced by the rehydration must therefore act early and effectively to eliminate the motivation to drink before TBW is fully restored to normal. The present experiment examined the interactions between active primary drinking following water deprivation and the ensuing physiological changes resulting from it. Changes of fluid distribution between the body’s cellular and extracellular compartments were estimated indirectly from sequential venous blood samples. The drinking pattern following 72 h water deprivation occurred in two distinct phases. The fist phase consisted of an initial intense burst of drinking which was completed within a period of l-4 mins. The cessation of primary drinking preceded any marked detectable changes in the blood parameters measured, thus suggesting that oral-gastric factors are important in controlling intake in the early stages of rehydration. Na+ and osmolality values fell below control levels within 20 min, thereby suggesting rapid rehydration of the cellular phase and indicating that rapid absorption of ingested water had taken place. However, haematocrit and plasma protein concentration levels, and therefore by implication the extracellular fluid deficit, remained above the control levels for the whole duration of the rehydration period. The evidence obtained in the present experiments, therefore, taken together with the weight of evidence presented by recent studies in the rat, suggests that termination of drinking is due to cellular hydration rather than removal of the excitatory stimulus resulting from the osmotic dilution which immediately follows water absorption. INCOMPLETE NURSING IN THE SOW T.S. WHATSON Edinburgh School of Agriculture, West Mains Road, Edinburgh (Gt. Britain)