Pulmonary hypertension in chronic pulmonary disease is due mainly to the impairment of the pulmonary vascular bed. It is still doubtful, however, whether neuro-humoral factors also participate in the genesis of this hypertension. Because of the difficulties of the methods that have to be used in the study of this problem, little is known about the autonomous regulation of the lesser circulation in man. Lately there have been some attempts to extend our knowledge in this field by pharmacological investigations of the lesser circulation.Most of the authors have dealt with pulmonary hypertension in congenital or acquired heart disease. Several substances have been employed as e.g. TEAB (Davies et al., 1954;Scott et al., 1955), hexamethonium (Gilmore et al., 1952;Judson et al., 1954;Wade et al., 1956) Rudolph et al., 1958).The genesis of hypertension in congenital and acquired heart disease, however, differs from that of hypertension in cor pulmonale. Little attention has been paid to pulmonary hypertension in cor pulmonale so that the number of studies concerned with the pharmacology of this problem is small. The only substances to be used in sufficiently large groups of subjects with cor pulmonale have been TEAB (Fowler et al., 1950) and hexamethonium (Malamos et al., 1957; Sancetta, 1955).However, results are difficult to assess since the doses of these substances used lowered systemic blood pressure. An attempt was therefore made to affect pulmonary hypertension in cor pulmonale pharmacologically by employing a substance that would lower pulmonary arterial blood pressure without substantially affecting systemic blood pressure. Since it was found (Widimsky et al., 1959) that priscol fulfils these requirements, the effect of this drug upon patients with cor pulmonale has been investigated in greater detail.Methods. Fifteen patients with pulmonary disease, 11 of these with pulmonary hypertension at rest, within an age range of 21-68 years were investigated by catheterization of the pulmonary circulation. One day before catheterization the following premedication was applied: phenobarbital 0-06 g. natr. bromati 0-6 g., chinidini sulphur 0-2 g., and on the day of catheterization phenobarbital 0 03 g., natrii bromati 0 3 g., and chinidini sulphur 0-1 g. were administered. Catheterization was performed in the usual manner, always in the morning.