Antipsychotics are a first-line treatment for psychotic disorders. The cytochrome P450 isoenzymes CYP3A4/5 and CYP2D6 metabolize most antipsychotics. The activity of these isoenzymes in children changes with maturation, so it is different from that in adults. Objective: to study the associations of CYP3A and CYP2D6 isoenzyme activity parameters with the efficacy and safety of antipsychotics in adolescents with an acute psychotic episode. Patients and methods. The investigation enrolled 53 adolescents with an acute psychotic episode who took antipsychotics. The observation period lasted 14 days. The CGAS, PANSS, UKU SERS, SAS, and BARS scales were used to evaluate the efficiency and safety of the therapy on day 14. The activity of CYP3A and CYP2D6 isoenzymes was measured by determining the metabolic ratios of the concentrations of endogenous substrates of the isoenzymes and their metabolites in a morning urine sample on days 1 and 14 of the study. The activity of CYP3A was assessed by the 6-beta hydroxycortisol/cortisol ratio; that of CYP2D6 was measured by the 6-hydroxy-1,2,3,4-tetrahydro-beta-carboline/pinoline ratio. The influence of carriage of polymorphic variants CYP2D6*4,*9,*10, CYP3A4*22, CYP3A5*3 on the activity of isoenzymes was excluded by removing their carriers from the analysis. Results and discussion. The investigators revealed an association of certain antipsychotic-induced undesirable symptoms with CYP3A and CYP2D6 activity parameters. On day 1, a lower CYP2D6 activity was initially observed in patients with tremor (0.54 [0.34; 1.34] vs 1.25 [0.91; 1.75]; p=0.023). Also, patients with any documented adverse reaction (ARs) to therapy had initially a decreased CYP3A activity (1.2 [0.85; 2.29] vs 2.55 [1.44; 4.83]; p=0.047) and an enhanced CYP3A activity during therapy (the activity difference between day 14 and day 1 was 0.28 [-0.28; 2.32] vs -1.3 [-3.47; 0.66]; p=0.042). Conclusion. The initially reduced activity of CYP2D6 and CYP3A isoenzymes is a significant predictor of antipsychotic-induced ARs in adolescents with an acute psychotic episode. The predictive role of CYP2D6 and CYP3A activity levels in the efficacy of antipsychotics has not been confirmed.