OBJECTIVE--To develop a method to measure end expiratory nitric oxide concentration and to use it to determine whether there are sex related differences between healthy men and women. DESIGN--Modification of commercially available chemiluminescent analyser to allow endogenous nitric oxide (endothelium derived relaxing factor) to be measured in a single slow forced vital capacity manoeuvre with evaluation of between and within subject variability. Serial measurements in women throughout the menstrual cycle. SETTING--Specialist referral hospital. SUBJECTS--59 healthy volunteers (40 men and 19 women). RESULTS--Mean (SD) peak expiratory concentrations of nitric oxide in men (75 (20) ppb (parts per billion)), and women (70 (37) ppb (first measurement)) were > 5 times those previously recorded. In women concentrations at midcycle (150(39) ppb) were significantly higher (mean difference 94.9(41), P < 0.001) than during menstruation (59(25) ppb). CONCLUSION--Nitric oxide production (endothelium derived relaxing factor activity) is influenced by cyclical hormonal changes in women. There was an increase of more than 100% at mid cycle. This may be pertinent to the lower risk of cardiovascular disease in premenopausal women.