The aim was to determine the influence of melatonin on basal levels of glucose (BG), activities of glucose- 6-phosphate dehydrogenase (G6PD), pyruvate kinase (PK), glucose-6-phosphatase (G-6-Ph) and lactate dehydrogenase (LDH) in the kidney of alloxan diabetic rats under conditions of different photoperiod. Alloxan diabetes was evoked via injecting the rats with a 5% solution of alloxan monohydrate intraperitoneally in a dose of 170 mg/kg of body weight. Three groups of animals were kept in conditions of artificial equinox, constant darkness and constant light respectively during 7 days. In each group there were subgroups collected for control, rats with diabetes mellitus (DM) – BG≥8.0 mmol/l, rats with impaired glucose tolerance (IGT) – BG≤6.9 mmol/l and DM and IGT rats which respectively received melatonin in a dose of 10 mg/ kg of body weight. Alloxan monohydrate administration results in a significant elevation of the activities of G-6-Ph by 147% and LDH by 39%, however a decrease of G6PD by 48% and PK by 67% were in a direct dependence by the presence of hyperglycemia. The established changes of the indices turned out to be more marked under the conditions of constant light. Introduction of melatonin led to improvement of the carbohydrate metabolism.