The adrenocortical gland is one of the most vascularized organs of the mammalian body. It undergoes continuous morphological changes dynamically dependent upon special permeability conditions related to various physiological and physiopathological events. The adrenal cortex of different adult mammals (3 pigs, 2 mice, 3 sheep) was studied by means of scanning (SEM) and transmission (TEM) electron microscopy. As seen by SEM it is formed of polyhedral cells which delimit a characteristic continuous labyrinth system of intercellular lacunae occupied by sinusoid-like capillaries. The capillary wall is fenestrated and is lined by flattened endothelial cells with their nuclear part bulging into the capillary lumen. The fenestrae are round or oval pores measuring 50-100 nm in diameter. They are usually clustered to form sieve plates and characteristically present a thin membranous diaphragm. Irregular microelevations and dome-like projections are often seen. These structures are made of thin cytoplasmic plates interrupted by numerous small pores which show a sieve plate profile. The pored-domes on the nuclear portion seem to be identical in structure to those found in the thinner endothelial part; some of them appear to detach from the endothelial cell. The pored-domes are structurally comparable to those reported both in the renal glomerular endothelium and in liver sinusoids. These endothelial structures may be an expression of the high rate of filtration of these tissues and may be also related to the final step of the replacement of the sieve plates during endothelial regeneration.