The letter deals with a theoretical analysis of the different currents originating in an M.I.S. structure for a heavy inversion region of operation, when a transient low-level electrical perturbation is applied. The expression for the generation-recombination current in the space-charge region is fairly different from that generally used; it is emphasised that the recombination rate is not constant and not equal to ni/2τn in the whole thickness of the transition zone. © 1969, The Institution of Electrical Engineers. All rights reserved.