1. A satisfactory method for the direct determination of the pulmonary arterial pressure in rats is described.
2. The arithmetical mean of the blood pressure in the pulmonary artery in a series of thirty-four normal albino rats under nembutal anesthesia is 256 mm. H20 (18.8 mm. Hg).
3. Intravenous epinephrine causes an abrupt but briefly sustained rise in the pulmonary arterial pressure with a gradual return to normal.