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.