Objective. To evaluate a role of melatonin deficiency in the metabolic syndrome and chronic cerebral ischemia (CCI). Material and methods. One hundred and seventy-nine men, aged 45—75 years, with CCI and components of metabolic syndrome (MS) were examined. Intima-media thickness (IMT) in the carotid arteries and cerebrovascular reactivity was assessed by the results of duplex scanning and hypercapnic test. Cholesterol spectrum parameters, glucose metabolism, inflammatory mediators, markers of lipid metabolism were measured in the blood serum. Daily changes in melatonin production were determined by measuring 6-SMT, a melatonin metabolite, in morning, evening and night urine. Results and conclusion. The levels of total 6-SMT excretion were 17.2 [10.01; 36.8] mcg/day. Patients with the values <10.01 and >36.8 mcg/day significantly differed by the PAI-1, leptin/adiponectin ratio, IMT and cerebrovascular activity. The combination of 4 or 5 MS components was identified in 19 (42%) patients with low 6-SMT excretion and only in 7 (15%) with high excretion (OR=3.9; 95% CI 1.3—12.2; p=0.01). Thus, the low level of endogenic melatonin and disturbances of circadian dynamics of its synthesis with the decrease in the night and increase in the evening portions are the risk factors for MS and CCI. Addition of melatonin in the complex treatment of CCI associated with MS and the low level of endogenic melatonin is pathogenetically explained.