Background: The hypothalamo-pituitary-adrenal (HPA) axis is a major stress responsive system in humans. Although there are numerous ways of testing responsiveness of the HPA in experimental animals, this is much more difficult in man. Hypercapnea is a very stressful stimulus for humans and has been used as an anxiogenic probe in psychiatric patients. We have now investigated whether the simple challenge of a single 35% inhalation of CO2 activates the neuroendocrine system as evidenced by changes in HPA; activity, as well as cardiovascular and subjective responses, in healthy volunteers.Methods: Fourteen healthy male volunteers were recruited. They underwent single vital capacity inhalation of room air and 35% CO2, in a single blind fashion. Neuroendocrine, cardiovascular and subjective fear measures were taken at regular intervals.Results: CO2 inhalation produced significant activation of the HPA axis in all subjects, as measured with plasma cortisol. Heart rate was decreased and systolic blood pressure was significantly increased shortly after the inhalation of CO2. The subjects reported short-lived symptoms of fear with the experimental gas.Conclusions: Single vital capacity inhalation of 35% CO2 activated the HPA axis in healthy volunteers. It also had a significant cardiovascular and psychological (anxiogenic) effect, as expected from previous published studies. The test is potentially useful in studying the responsivity of the HPA axis in health and disease. (C) 2002 Elsevier Science Ltd. All rights reserved.