Food and water intake were restricted by 29% for 48 h in ovariectomized ewes, to see whether such changes could cause a pattern of water and electrolyte excretion similar to that observed during the oestrous cycle. The closest resemblance appeared in the effect on water balance and there was also a rise in sodium excretion, as at oestrus. Restriction of intake failed to duplicate the marked sodium retention or, more particularly, the persistent potassium retention which follow natural oestrus. Restriction of food alone produced changes less similar to those seen at oestrus, mainly because this treatment caused a large increase in urine output. The similarity between the effects of restriction and 'fasting natriuresis' is discussed, with particular reference to the mildness of the stimulus in these experiments. Indications that the gut may be as important as the kidney in the regulation of salt and water balance are discussed in relation to other herbivores and to man.