Bitches on a fixed food intake were kept in metabolism cages. Urinary volume, drinking and body weight were measured each day and loss of water by evaporation calculated. The mean evaporative loss of individual dogs varied from 8 to 116 g/kg/day; this could be accounted for by differences in the behaviour of the dogs resulting in different amounts of activity in the cages. Evaporative loss also varied widely from day to day. In direct observations in the laboratory evaporative loss of dogs at rest was about 1 g/kg/hr and this was increased by mild exercise to about 5 g/kg/hr. The environmental conditions were never warm enough to significantly affect evaporative loss. The amount of water drunk by each dog varied from 4 to 109 ml./kg/day; the active dogs with high evaporative loss drank most water, the water intake thus approximately matching evaporative loss. Details of the drinking were recorded by a meter. Much of the drinking occurred during the times of the day when the animals were most active, and consisted of small drinks of 30-100 ml. Water given by stomach tube inhibited drinking for as long as 23 hr. A period of water deprivation increased drinking, usually in the form of large drinks of over 100 ml. The mean urine volume varied in different periods of observation from 80-380 ml./day; the urine was of high concentration, about 1600 m-osmole/l. The volume was not related to activity of the dogs and varied little from day to day. The main determinant of the volume of the urine appeared to be the rate of excretion of solutes which varied very little because of the fixed daily intake of food. There were only a few days on which the urine was of high volume and dilute; in discussion it is stated that this means that varying release of antidiuretic hormone played little part in determining the urine volume and in the regulation of water balance. A small dose of water (about 5 ml./kg body weight) given by stomach tube always prevented drinking; a larger dose was required to produce water diuresis. This explains why water diuresis rarely occurred in dogs drinking naturally. Water balance was achieved because the evaporative loss due to variable activity was matched by drinking.
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