The measurement of exchangeable potassium in living pigs and its relation to body composition.

I . Twenty pigs were rcared to a weight of 90 kg on various dietary regimens so that their lipid content varied from 16 to 38%. They were injected intravenously with 400pCi of ‘*KCI. T h e animals were killed 28 h after injection and their contents of water, lipid, sodium and potassium were determined. 2. The specific activity of plasma reached equilibrium 1-12 h after injection, that of urine not until 20 h after. 3. Losses of activity in urine, faeces and gut contents in the 24 h after injection amounted to 3.1, 0.3 and 1 .5 :h of the injected dose, respectively. 4. Total body K was more accurately estimated from urine specific activity than from plasma specific activity. 5. Exchangeable K, estimated from urine specific activity Kec,), was highly correlated with the weight of fat-free tissue (I = 0.93), the residual standard deviation of the regression being 1.8 kg fat-free tissue. 6. Percentage extractable fat was equally well correlated with Kec,, (r = -0.92).