Followings are the results of studies on megakaryocytes in the spleen of the rodent of the successive stages from the embryonic stage up to the adult. 1) In the rodent, megakaryocytes are always found in the spleen from the embryonic stage up to the adult, especially in large numbers in the infant. Among the rodent, the largest numbers of megakaryocytes are found in the spleen of mice, less in rats, and only very small numbers in guinea pigs and rabbits. 2) No significant morphological differences are noted between megakaryocytes in the spleen and those in the bone marrow. 3) By splenic tissue culture method, megakaryocytes in the spleen are found to have deformated movement, pseudopodial movement, and tentacle formation for the platelet separation as those in the bone marrow. 4) Megakaryocytes in the spleen of mice and rats have motility and ability of platelet formation from the embryonic up to the infant stage, but these functions are found to be decreased during the adolescent and adult life. In guinea pigs and rabbits, the functions of megakaryocytes in the spleen are decreased after the mid-embryonic stage. 5) A correlation is noted between the functions of the megakaryocytes and erythropoiesis in the spleen, a rise and fall of the one being accompanide by the other.