Numerous hypotheses have been proposed about the pathogenesis of the polycystic ovarian syndrome (PCOS). However, hormonal control of persistent follicles has not been established. The objective of the present study was to compare the follicular structure and hormonal profiles of rats treated with the adrenocorticotrophic hormone (ACTH) with two experimental models of PCOS. ACTH-treated animals were compared with those exposed to continuous light, those treated with estradiol valerate, and with control (in proestrous and diestrous). Serum hormone levels, histomorphometrical changes, and immunoexpression of vimentin, cytokeratins, cadherins, and proliferating cell nuclear antigen (PCNA) were examined. Treatment with ACTH resulted in an elevation of corticosterone secretion with LH reduction but without changes in ovarian morphology. Although stress (or ACTH) stimulation may be only one of pathophysiological mechanisms involved in follicular cyst pathogenesis in other species, we do not have important evidence to suppose that this would happen in rats.