The results of the study of certain biochemical parameters of blood in grey seals (Halichoerus grypus Fabricius,
1791) from birth to weaning are presented. In the blood plasma, 20 indices describing the state of the metabolism
of proteins, carbohydrates, lipids, minerals (total protein, urea, creatinine, glucose, lactic acid, total
lipids, triglycerides, cholesterol, calcium, phosphorus, sodium, potassium, magnesium, iron, aspartate aminotransferase,
alanine aminotransferase, γ-glutamyltransferase, creatine kinase, alkaline phosphatase, lactate
dehydrogenase) are studied. It has been established that metabolic changes in the early period of postnatal development
of grey seals occur in mammals in general, but the expression level of individual reactions may be
considered a feature of pinnipeds. By the ratio of transaminase activity, the predominance of catabolism over
anabolism was established in animals in the studied period of development. The most significant changes
in the metabolism of grey seals were noted during weaning and the transition to self-feeding. The adsorption-
transport function of erythrocytes was evaluated by washout on the erythrocyte membranes using 3%
sodium chloride solution. It was established that all detected components of blood plasma are found in erythrocyte
washings. The level of adsorption, expressed as a percentage of the content of a certain metabolite
in the blood plasma, is not the same for different compounds. Enzymes, urea, creatinine, lipids are adsorbed
most on the erythrocyte membranes (in some cases more than 100%), and glucose, lactic acid, cholesterol,
iron, calcium are adsorbed in a smaller quantity (<50%). The data obtained indicate that the transfer of organic
compounds on erythrocytes is more variable and demonstrative than shifts in the corresponding blood
plasma parameters, and erythrocytes can participate in maintaining a range of concentrations of a number of
substances in the blood plasma.