The plasma concentrations of adrenocorticotropic hormone (ACTH) in rats pretreated with capsaicin as neonates were compared with those of control rats pretreated with the capsaicin vehicle. Capsaicin pretreatment has been shown earlier to abolish the increase in plasma ACTH concentration induced by cold stress while not affecting that induced by restraint stress. In the present experiments rats pretreated with capsaicin showed the same increase in plasma ACTH concentration in response to an i.v. infusion of ovine-corticotropin releasing factor as control rats pretreated with the capsaicin vehicle. Intraperitoneal injection of formalin, surgical stress and intravenous infusion of (-)-isoprenaline increased plasma ACTH concentrations in control rats. In capsaicin pretreated rats the increase in plasma ACTH was significantly attenuated. It is concluded that capsaicin-sensitive sensory neurones mediate the activation of pituitary ACTH secretion in response to somatosensory stimuli. The function of the corticotroph cells of the anterior pituitary is not impaired by capsaicin treatment.