Plasma Atrial Natriuretic Peptide (ANP) levels are elevated in patients wish chronic obstructive pulmonary disease (COPD) and can play a role in oedema formation. Plasma ANP levels measured in 60 patients with COPD were compared with results of pulmonary haemodynamics, with therapeutic response to treatment in the time of exacerbation of COPD and with physiologic status of patients with stable COPD. Plasma ANP levels did not correlate with right atrial and pulmonary artery pressure, but were significantly related with right ventricular end diastolic volume and with right ventricular wall volume measured by magnetic resonance imaging. Oxygen breathing (2 l/min by nasal prongs in 30 min.) did not change either mean pulmonary artery pressure or ANP levels. Among patients studied during an acute exacerbation of COPD, plasma ANP levels were higher in patients with oedema (302 +/- 185 pg/ml) than in patients without oedema (87 +/- 43 pg/ml). Oxygen therapy applied in one hour did not influence plasma ANP levels. Plasma ANP levels decreased during first three days of treatment in patients with oedema. This decrease was related to the body weight. In the group of patients with hypoxemia in stable COPD, plasma ANP levels (120 +/- 50 pg/ml) were higher in patients with the history of hypercapnia and oedema than in others (54 +/- 15 pg/ml).