The research objective was to study histological structure of the spleen of the Baikal seal in onto-genesis. The research problems were to specify spleen microstructure in the Baikal seal in post-natal ontogenesis and to reveal morphometric indi-cators of internal structures of the spleen of the Baikal seal. The researches were conducted on the basis of Department of Anatomy, Physiology and Microbiology of FB EI HE ‘Irkutsk SAU’ named after A.A. Ezhevsky. The object of research was the Baikal seal got in the Kabansky area of the Repub-lic of Buryatia within R & D Program approved by the Russian Fisheries Ministry for 2015. As material for studying the spleen from the species aged from 5 months till 12 years of age (n=12) served. The age of animals was determined by the rings of den-tine on the basis of canine and by the horn rings of the claw. Distinctive feature of histological structure of the spleen in the Baikal seal is the existence of dense connective tissue capsule, in immature ani-mals it increases by 1.19 times and in mature ani-mals - by 1.38 times in relation to cumutkins. Tra-beculas have different extent and width increasing in immature animals by 27.8 % and mature animals for 34 % in comparison with cumutkins that is con-firmed with correlation coefficient between age and width of trabeculas, between the length of spleen and width of trabeculas. In white pulp the border of marginal zone is distinctly expressed. In red pulp there are a large number of the erythrocytes giving to the spleen specific color, such structure is char-acteristic for all studied age groups. The area of white pulp increases by 1.3 times in immature indi-viduals and by 1.4 times in comparison with cu-mutkins with age. The area of red pulp considerably prevails over white pulp in all age groups and makes in cumutkins 11880.0±567.8 mkm2, in im-mature - 13748.5±492.6 mkm2 and in mature - 15180±613.2 mkm2.