The paper discusses physiological and biochemical aspects of relations in the "parasite-host" system by the example of market Hy-Line laying hens and Dermanyssus gallinae. The hens of the experimental group were kept in a poultry house with a degree of D. gallinae infection at "+++"; this condition was considered as a stress factor for the birds. The control group had no parasitic agents. Blood was taken from 10 randomly selected birds from the experimental and control groups to determine physiological, biochemical and morphophysiological parameters. The complex of parameters was analyzed for carbohydrate-energy, protein and lipid metabolisms. The nature of metabolic processes in laying hens in dermanyssosis was assessed. The endocrine system supply of the hen’s organism with stress-associated hormones, in particular cortisol and triiodothyronine, was studied. The paper presents the results of molecular consequences of a stress reaction developed in hens in dermanyssosis. Stress in birds was accompanied by excessively activated lipid peroxidation, and a decrease in antioxidant defense. The trend to an increase in the content of Schiff’s bases, the end products of lipid peroxidation in the experimental hens, allows us to note the chronicity of stress reaction, which causes derangement of metabolism in hens in dermanyssosis. The third stage of stress reaction, the stage of exhaustion, was diagnosed in laying hens in dermanyssosis.