I n technical microbiology microorganisms often undergo alternating milieu changes. For instance, this is the case in recirculation reactors. The organisms react on these changes with an increase of entropy production and in connection with this with increasing substrate consumption. This increasing substrate consumption contradicts the aim of an optimum yield from a given substrate. Thus, studies of the reaction of microbial growth t o alternating milieu changes are of great importance.A simple model of the influence of alternating milieu changes on specific substrate consumption is given. I n i t the biological conversion of substances is built up by an irreversible consecutive reaction.After preliminary examinations on the analogue computer the reaction of the model to alternating perturbations is simulated on the digital computer. The results of the simulations are conipared with experimental data.The good agreement of experiment and model justifies the use of the simple formulation in the preparation of technical processes.