In experiments on the nitrogen metabolism of piglets during the first days of life (McCance & Widdowson, 1956) some of the piglets were given an evaporated cow's milk mixture. This contained more sodium, chloride and potassium than sow's milk, and piglets having it developed hypertonic expansion of the extracellular fluids. This was more fully investigated by McCance & Widdowson (1957), and it was then shown that human infants reacted in a very similar way. The animals in the first experiments also retained more potassium from the cow's milk mixture than they appeared to require for co-ordinated growth. The interest of this is not confined to piglets, for something similar has been shown to take place in human infants (M. 0. Beem & C. A. Smith, personal communication). Further experiments therefore were designed to study this retention of potassium under more controlled conditions than were possible in 1956. The experiments were planned on the lines of those carried out on sodium chloride loading and are reported in this paper.
METHODSThe general arrangements were the same as those described by McCance & Widdowson (1956 and only special points need be described in detail. Three litters have been used. Each has been treated in an almost identical manner and all have given similar results. The first piglet in each litter was given water, and the second an equal volume of water to which KCI (120 m-equiv/l.) had been added. This contained the same concentration of chloride as the 0 7 % NaCI solution used in the previous experiments (McCance & Widdowson, 1957). The third piglet was given a volume of sow's milk times as great as the volume of water. This made the intakes of water about equal since the sow's milk used contained 15-20% of solids. The fourth piglet was given a volume of sow's milk equal to that given to the third, but KCI was added to make the concentration of potassium in the water of the milk equal to the concentration of potassium in the water given to the second piglet. This made the potassium intake of piglets two and four about the same and roughly three times that of the third piglet, which was having sow's milk. These piglets were fed two-hourly as before. The fifth piglet was given nothing.The experiments, like the ones on sodium chloride, lasted for 40 hr and the urines were again