AbstractPatients with impaired renal function often surfer undesired side- effects of drugs caused, above all, by reduced elimination of unchanged dmg and toxic metabolites as well as by diminution of the distribution volume. Plasmapheresis is one well-known treatment procf'dnre for such patif'nt.s, but may reduce drug plasma concentrations below the toxic and possibly below the therapeutic level, and therefore an optimal individual drug therapy design is needed. This work (kals with the development of a time-varying multicompartment model for the evaluation of the inflnences of plasmapheresis on drug pharmacokinetics. Analog- hybrid computer was chosen as the simulation tool. Model validation was realized by the aid of the corresponding in vivo clinical measurements and subsequent chemical analysis of the sampled biological materials. The approach is illustrated by a particular drug pharmacokinetic st.udy under the conditions of plasmnpheresis. The obtained result show the suitability of the chosen modelling techniques as well as the flexibility of the used simulation tool in designing the mentioned therapy. The approach has been already used for some problematic cases. With the use of the appropriate digital simulation language on IBM PC/AT computer, the procedure will soon become available in routinc clinical practice.