The regularities of the structural and functional changes of the male red deer (maral) adrenal in the postnatal ontogenetic period are determined with the help of morphological and histochemical indices. It was found that the adrenal gland of newborn red deer had immature morphological structure with the signs of weak physical activity. The most active growth of the cortical and adrenal medulla substance coincides with the period of intensive growth of the whole organism up to the age of six months. The full structural gland formation takes place at the age of one-year and six months. The highest functional condition of cortical adrenal substance in the studied age-grade is typical for the adult animals, and the highest adrenal medulla gland substance is typical for the animals in the period of sexual ripening, that is two years and six months. The morphological reasons of the functional adrenal activity reduction with the simultaneous development of compensatory mechanisms in the cortex take place at the later stages of postnatal development.