The effect of acute i.v. administration of diltiazem on pulmonary haemodynamics was compared to that produced by oxygen in 10 hypoxemic patients with chronic obstructive lung disease and pulmonary hypertension (PAPm greater than 20 mmHg), without left ventricular dysfunction. Determinations were carried out at baseline, during 100% oxygen inhalation, at least 20 minutes after oxygen withdrawal and 15 minutes after i.v. diltiazem loading dose of 0.25 mg/kg followed by the infusion of 1 mcg/kg/min. Oxygen inhalation produced no significant modifications of haemodynamic variables, whilst a significant (p less than 0.05) decrease of PAPm, pulmonary arteriolar resistance (PAR) and peripheral resistance (TPR) was observed after diltiazem administration (respectively 14%, 23% and 11.6%). Heart rate, cardiac index and blood pressure did not change significantly even with diltiazem. These results could support the inclusion of diltiazem in the therapeutic regimen of pulmonary hypertension due to chronic obstructive lung disease, but further investigations are needed to clarify the predictive value of its acute administration in assessing long term efficacy.