The present study shows that various metabolic substrates are of different importance for the maintenance of the osmolality of the body fluids. Many hormones seem to influence the osmolal balance either by influencing the number of particles in a solution or the water balance. The effect of one particular hormone on osmolality can often be difficult to establish, because one effect will start a sequence of regulatory processes.Measurements of plasma and urine osmolality will therefore give a more varied picture of the condition in the body than, for instance, studies of fluid and electrolyte balance alone.