Objective — to study the level of daily melatonin excretion in adolescents with type 1 diabetes mellitus (DM1) at the stages of puberty, taking into account the course of puberty. Materials and methods. The investigation involved 172 patients with DM1 aged 9—18 years (85 boys and 87 girls), who stayed in the Endocrinology Department of the SI «Institute for Children and Adolescents Health Care of the NAMS of Ukraine» for examinations and treatment. The DM1 duration was 1 to 10 years. With account of the level of sexual development at the time of examination, adolescents were divided into the following subgroups: prepuberty (T1); early puberty (T2); puberty itself (T3); late puberty (T4). The nature of sexual maturation was assessed with account of the norms for the corresponding age. The nature of melatonin production was assessed by the excretion of its main metabolite — 6-sulfatoxymelatonin in daily urine, which was determined by fluorometric method. Statistical processing of the results was carried out using application packages Microsoft Excel and SPSS 17.0. Results and discussion. The effects of puberty on melatonin production have been determined, with its increase during puberty and late puberty. It has been proven that violation of the puberty course is accompanied with changes in the melatonin levels, corresponding to the degree of sexual development of girls and boys with DM1. In conditions of accelerated puberty, the melatonin level increases, and it decreases in case of slowdown sexual development. In girls of 14—18 years old, the lowest melatonin levels were determined in patients with delayed sexual development and primary amenorrhea. During sexual maturation, formation of epiphysealpituitarygonadal and epiphysealpituitarythyroid links takes place, their nature changes in the course puberty. Conclusions. In adolescents with DM1, the age-related dynamics of daily melatonin excretion during puberty is preserved, with an increase in its levels during late puberty. Melatonin levels in patients with DM1 depend on the sexual maturation course, and they are altered in case of its disorders. Regardless of gender, at delayed sexual development significantly lower melatonin levels were registered in comparison with patients with normal sexual development. The nature of correlations between melatonin and hormonal indicators depends on the level of an adolescent’s sexual development and gender.