In parallel with the light microscopic observations, the change of thiamine content in reference to the correlation between the free and ester types of thiamine was investigated during the seed development (from pollination to maturation) of wheat and maize. With respect to the correlation between the content of the free and ester types of thiamine, the following 4 different phases were distinguished in the process of seed development. The first phase is characterized by the presence of the ester type of thiamine only, and the second phase by the preponderance of the ester type. The third phase is marked by an increasing amount of free thiamine, and the fourth phase by an exceeding amount of free thiamine. At the end of the second phase, the relative amount of free thiamine and its ester are reversed. Differentiation of embryo comes nearly to an end by this period. Thiamine triphosphate can be detected until the second phase. It seems that the appearance and disappearance of thiamine triphosphate during seed development are in closer conjunction with embryonal differentiation.