The aim of this study was to investigate the development and diffferentiation of the adrenal glands in the grass snake during early stages of ontogenesis. After light microscopy and ultrastructural investigation, six developmental phases were distinguished in the grass snake adrenal glands. Similar developmental phases were described in pig embryos but they slightly differ from those presented above. In reptile and mammal embryos a similar direction of changes is observed in primaordia of both tissues forming adrenal glands.