In several previous experiments to determine the composition of inters
titial fluid, the results varied depending on the collecting technique
, and the electrolyte concentrations differed from those of a hypothet
ical ultrafiltrate of plasma. In our approach, since a change of posit
ion from standing to supine is accompanied by hemodilution with inters
titial fluid, we used the changes in hematocrit and composition of pla
sma in 20 subjects before and after lying down to calculate the compos
ition of added interstitial fluid. The estimated protein concentration
was 20.6 g/L, and the concentrations of total calcium and magnesium w
ere low, in accord with a lower concentration of protein-bound calcium
and magnesium. The activity of free cations was also lower, in agreem
ent with a Donnan equilibrium potential of 1 mV across the endothelium
. The concentration of leukocytes and platelets decreased according to
the hemodilution, implying no escape or mobilization of these element
s.