Aim. A study of the relationship between indicators of central nervous system functioning in adolescents and various types of Internet use: adaptive (AIU), non-adaptive (NIU) and pathological (PIU). Design. Comparative clinical study. Materials and methods. The study included 202 adolescents aged 12 to 17 years old (median age: 14 [13; 15] years old), pupils from general educational establishments of Abakhan (Republic of Khakassia) with various online behaviours: 86 (42.6%) boys and 116 (57.4%) girls. The observation groups were formed on the basis of their online behaviour: 71 (35.1%) adolescents with adaptive Internet use, 107 (53.0%) — with non-adaptive Internet use, and 24 (11.9%) — with abnormal Internet use. The examination program involved the study of the following parameters and characteristics: type of online behaviour, quantitative parameters of the simple sensomotor reaction to visual stimulation and its integral characteristics (levels of CNS activation, accuracy, speed and stability of reactions). The functional state of the CNS (optimal, satisfactory, unsatisfactory) was assessed using the characteristics of the simple sensomotor reaction to visual stimulation using Psikhofiziolog computer appliance. Results. In adolescents with PIU, a tendency was revealed to deteriorate certain indicators of the sensorimotor test, which was manifested by a decrease in concentration and stability of attention (in terms of error-freeness and stability) and slower reactions (in terms of speed of action). The results of the study indicate the predominance of inhibition processes in the central nervous system, a state of fatigue, a decrease in mental performance and cognitive activity in adolescents with PIU. Conclusion. The data obtained prove the relevance of the problem of pathological Internet use and its impact on health in the ontogenetic period under study, and indicate the need for its further study in order to optimize preventive and corrective measures in the teenage population of Internet users. Keywords: teenager, online behavior, central nervous system, functional state, visual-motor reaction, correction.