Pulmonary plasma kinetics of endogenous noradrenaline (NA) and tritium labelled L-noradrenaline (3H-NA) was studied in fifteen subjects during pulmonary arterial catheterization. Plasma NA concentration in femoral artery ranged from 0.5 to 8.2 nmol l-1, mean 2.3 nmol l-1, which was not significantly different from that of age-matched control subjects. The lungs extracted both endogenous NA and 3H-NA significantly, but no significant pulmonary extraction of endogenous adrenaline was found. The pulmonary arterial-systemic arterial extraction ratio of NA was mean 0.08 (n = 9) as compared to that of 3H-NA: mean 0.07 (n = 8, NS). Likewise mean pulmonary clearances of NA and 3H-NA were not significantly different (97 ml min-1 X M-2 v. 124 ml min-1 X M-2, NS). Estimated whole-body clearance of noradrenaline was mean 0.80 l min-1 X M-2 (n = 6) while the pulmonary clearance amounted to 19% of this value. The small, but significant, pulmonary extraction of circulating noradrenaline implies that whole-body clearance, as estimated from infusion rate and systemic arterial sampling, will be overestimated by approximately 7%. As pulmonary extraction of NA and 3H-NA was almost identical, the results indicate no significant pulmonary contribution to circulating noradrenaline.