Immunoreactive (IR) plasma prostaglandin (PG) levels were measured in samples collected simultaneously through catheters placed in the right ventricle and the thoracic aorta in fasting anaesthetized dogs. There were significantly greater levels of IRPGB (P < .01) and IRPGA (P < .05), but significantly less IRPGE (P < .01) in the aorta than in the ventricle. During femoral vein infusion of PGE1, PGB1 and PGA1, respectively, PGE1 was approximately 87% metabolized, but PGB1 and PGA1 were not degraded by the lung. There was no measurable increase in IRPGB or IRPGA levels in the thoracic aorta during intravenous PGE1 infusion. It was concluded that in the resting state PGE is actively degraded by the lung; that the lung very effectively degrades PGE1 but does not degrade PGB1 or PGA1 during infusion of these prostaglandins; and that pulmonary metabolism of PGA1 probably does not result in formation and release of PGB or PGA into the arterial circulation. Additionally, the possibility exists that in the resting stage PGB and/or PGA are actively secreted by the lung, but the immunoassay methodology used does not permit resolution of this point.