We report the light microscopic, transmission and scanning electron microscopic features of the adrenal cortices in rats bearing a medullary thyroid carcinoma cell line transfected with a corticotrophin-releasing hormone (CRH) cDNA expression vector. The animals had elevated CRH, ACTH and corticosterone blood levels, involuted thymuses and markedly enlarged adrenal glands with prominent lipid-depleted cortices and dilated congested capillaries, similar to those of animals treated with ACTH. Using electron microscopy it was found that the enlarged fasciculata and reticularis zones were composed of large, compact cells with abundant smooth endoplasmic reticulum, prominent Golgi complexes, increased number of large mitochondria with focal loss of cristae and cavitation of the internal compartments, numerous lysosomes and prominent elongated microvilli. In addition, small cytoplasmic fragments were seen within the capillary lumina; these structures resembled microvilli that were apparently detached from adrenocortical cells and entered the blood stream via discontinuous endothelium of dilated capillaries. By scanning electron microscopy it was found that the cells had bulging surfaces with scattered pits and numerous long microvilli pointing in different directions. This animal model allows analysis of the effects of protracted CRH excess resembling tumoural CRH-dependent Cushing's syndrome in human patients. Our findings call attention to the role of microvilli in adrenocortical secretion. The increased number and size of microvilli has been thought to lead to an increase in the surface area of adrenocortical cells, thereby facilitating hormone discharge. The detachment of microvilli from adrenocortical cells may represent a form of apocrine secretion and may contribute to hypercorticosteronaemia in CRH excess.