Melanin-concentrating hormone (MCH) is a peptide reported to inhibit ACTH and cortisol secretion in teleost fish. Its ability to modify the activity of rat corticotrophs, however, remains controversial. We report here that while the peripheral injection of rat (r) MCH failed to alter plasma ACTH levels of conscious rats with an intact blood-brain barrier (BBB), it significantly activated the hypothalamic-pituitary-adrenocortical axis of rats with increased BBB permeability induced by protamine sulfate administration into the internal carotid artery. Similarly, the intracerebroventricular injection of this peptide into rats with intact BBB measurably released ACTH. The ACTH response to rMCH was markedly, but not totally, inhibited by passive immunoneutralization of CRF. These results indicate that rMCH acts within the central nervous system to stimulate the hypothalamic-pituitary-adrenocortical axis of rats, and that the site of action of the peptide is located in brain structures protected by the BBB. Activation of CRF-secreting neurons represents an important final pathway, although other regulatory factors also seem to be involved.