If nonlinear biological processes are investigated by means of tracer experiments they can be modelled with linear kinetic equations (compartment equations) as long as the total system is in a stationary state. But if nonstationary behaviour is included considerations on the kinetics of the individual processes are necessary. Within the range of biological and agricultural investigations especially first order reactions (constant fraction processes), zero order reactions (constant amount processes) and saturation reactions (Michaelis-Menten-kinetics) are to be taken into account. A rigorous treatment of data based on system theory can be preceded by a graphic-algebraic procedure which may be more or less uncertain in its results but which can easily be handled.