The effects of atrial natriuretic factor (ANF) on the adrenal steroidogenesis were examined in the zona glomerulosa of rats in vivo. ANF administration provoked a significant decrease in the width of zona glomerulosa (P < 0.01) and a significant enlargement of nuclei of zona glomerulosa cells (P < 0.01). Ultrastructurally conspicuous change was observed in mitochondria of the cells of the zona glomerulosa treated with ANF, representing elongated mitochondria with "parallel arrays" of tubules with pipe-like structure. In vitro autoradiography 125I-labelled ANF binding displayed a significant decrease of a specific receptor site in the zona glomerulosa treated with ANF. A significant lowering in plasma levels of aldosterone was observed in ANF-treated rats. These findings could be interpreted as an inhibitory effect of ANF on adrenal steroidogenesis of the glomerular zone by action through a specific ANF-receptor.