The objective of the present review was to analyse the problems of expert evaluation of the results of forensic chemical investigations of clozapine in the biological material. Such an analysis is needed because many topical aspects of the quantitative evaluation of toxic clozapine concentrations remain unclear. The treatment with clozapine is associated with its accumulation in blood in concentrations up to 2 mg/l in the absence of any toxic effect allegedly due to the development of tolerability of this agent. In the tolerant patients the ratio of the main clozapine metabolite, norclozapine, to clozapine itself in the serum amounts to 0.6-0.9. This value falls down to 0.3-0.4 in case of acute intoxication. In the case of identification of other pharmaceutical products narcotic drugs together with clozapine their influence on the activity of enzymes responsible for clozapine biotransformation should be taken into consideration. The concomitant intake of clozapine and alcohol may be dangerous for the clozapine-intolerant subjects. It is concluded that the above observations must be borne in mind in the assessment of the results of forensic chemical analysis.