Histamine, mediates pulmonary allergic reactions that include airway bronchoconstriction. Human airways are considered to have H1, H2 and H3 receptors. Bronchoconstrictive effect of histamin is due to H1 receptor stimulation. H2 receptors possibly balance this effect by bronchodilatation. Since the incidence of peptic ulcer and gastroesophageal reflux is more common in patients with chronic obstructive pulmonary disease (COPD) than the normal population, H2 receptor blockers are used more extensively by COPD patients. To the best of our knowledge, there is no study in literature about the effects of H2 receptor antagonists on pulmonary function tests (PFT) of the patients having COPD. This study evaluated the effects of ranitidine (an H2 receptor antagonist) on PFT of the patients having COPD with peptic ulcer or gastroesophageal reflux and of healthy volunteers.Fifty mg. of Ranitidine was given intravenously to 27 COPD patients ( 23 men, 4 women) and 25 healthy volunteers (18 men, 7 women). PFT were applied before and 15, 30, 60, 120 minutes after the injection of Ranitidine. The decrements of FVC, FEV1 and FEF25-75 in the COPD patients that appeared 30 and 60 minutes after Ranitidine injection were statistically insignificant (p>0.05). There were not significant changes in the PFT of the healthy volunteers. The comparison of mean differences in FVC, FEV1 and FEF25-75 of the healthy volunteers and COPD patients did not reveal statistical significance (p>0.05). Since the minimal decreases in FEV1 and FVC due to usage of H2 receptor blockers may clinically worsen COPD patients who have severe obstruction in PFT; follow-up of them by measurements of PFT is advised. However, H2 receptorblockers may be safely used by the COPD patients who have mild or moderate obstruction in PFT.