The three zones of adrenal cortex are thought to arise from a single multipotential stem cell. Immunohistochemical studies of fetal and adult adrenals using an antibody against a previously-cloned ZOG protein, a rat homolog of Pref-1, were conducted to explore its roles in the differentiation of cortical tissues. At the early embryonic stage, ZOG was already expressed in adrenogonadal primordial cells. The ZOO-positive cells gradually formed the adrenal primordium by E14.5. By E17.5 the expression was repressed in the inner part of the aggregate and these cells began to express CYP11B1. The ZOO-positive cells at this stage existed at the periphery of the aggregate but they did not express CYP11B2 yet. Not until E20.5 did the aldosteronogenic cells appear among the ZOO-positive cells at the outermost part of the gland. Based on these and the other findings the zonal development of the adrenal cortex is discussed.