Dynamic equilibrium in a biological system implies that the compartment under study does not change in size during the period of observation. In many biological systems there are, however, net changes with time and this report deals with the mathematical treatment necessary to calculate unequal rates of inflow and outflow.

A method is presented for the calculation of transfer rates in a two compartment system when the rates of flow between these compartments are unequal but constant. Equations were developed to calculate the amount of material transported per unit time derived from measurements of specific activity and compartment size.

The problems of (1) sampling from the pool and (2) the effects of analytical errors on the estimation of rate have been evaluated. An example has been presented in which the derived equations have been applied to a study of the simultaneous passage of sodium into and out of a permanently isolated loop of bowel.

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