Abstract
A set of simple differential equations have been developed to describe the movement of isotopes across barriers in biological systems. These equations cover the following examples: (a) uptake of stable isotopes from a constant source as shown by the uptake of deuterium in maternal and fetal rat cholesterol; (b) uptake of K42 by red blood cells from dilute plasma soln.; (c) flow of Na out of the plasma in man; (d) uptake of Na from the plasma into the cerebrospinal fluid of man. There is a short discussion on the possibility of differentiating between ultra-filtration and secretion, and also on the criteria for precursors.