THE AIM. To study the key homeostatic physicochemical parameters of blood serum characterizing the patient's state with COVID-19 of varying severity. Patients and methods. The study involved 94 patients with COVID-19, age from 24 to 102 years, median - 67 years, men - 43, women - 51. Patients were divided into 3 groups: 1st - 40 patients with a relatively mild course, 2nd - 22 patients with a severe course of the disease, after treatment they were discharged from the clinic, the 3rd - 32 patients with a very severe course of COVID-19 and a fatal outcome. The concentration of ions in blood serum was measured on a Gem Premier 3000 (Instrumentation Laboratory, USA), clinical analysis was performed on a BC-5380c Mindray hematology analyzer (Mindray, China), biochemical studies were performed on an Architech c4000 analyzer (Abbott Laboratories, USA). RESULTS. The harbingers of a serious impairment in patients with COVID-19 were an increase in the Na+/K+ ratio in blood serum from 32,7±0,8 to 44,7 ± 2.1 (p