Tracer influx kinetics have been analysed theoretically to determine the size of the transport pool with no assumptions regarding the transport pathway. 2. For a calculation of the size of the transport pool to be made, the following six conditions are required by the theory: tracer steady state attained, tissue steady state attained, Isc measures next flux, small magnitude and constant time-course of efflux, and correction for decay in pumping rate. 3. The size of the pool, SI, is given by the steady state influx, Finfinity, divided by the mixing-time constant, alpha. 4. Some experimental results are analysed by three different graphical methods, and it is shown that these three methods are equivalent. Specifically, alpha is equal to the reciprocal of the 75% mixing time, t75, divided by 1n 4 and is equal to the reciprocal of the lag time, X. 5. The tracer kinetic theory is applied to active potassium transport across the isolated short-circuited midgut: the transport meets the six conditions required by the theory. 6. The size of the transport pool of potassium in one midgut is calculated to be 80.5 muequiv./g wet weight under high-K steady-state conditions. A value as high as this suggests that the pool is intracellular.
