A circuit for the electrical simulation of transient diffusion and flow of material between two volumes is described. It allows such processes to be simulated even when forward and reverse transfer rates between the two volumes are unequal. When the reverse rate is zero, the circuit functions as a very‐high‐input‐impedance integrator of wide dynamic range and high accuracy. The circuit will integrate the current through a given resistor, even when this resistor is time varying or nonlinear. For voltage integration, it can have a minimum input impedance approaching 109ohms.