The single-bolus multiple-indicator-dilution technique was used to evaluate pulmonary removal of tracer 125I-labeled endothelin-1 in seven anesthetized dogs. Simultaneously, pulmonary arterial and aortic blood samples were obtained and assayed to determine the levels of immunoreactive endothelin-1. When 125I-endothelin-1 was compared with a plasma vascular reference (Evans blue dye), there was a single passage mean extraction of 31 +/- 8%. In contrast, there was no significant difference between immunoreactive endothelin-1 levels measured in blood samples from the pulmonary artery and the aorta (1.26 +/- 0.58 and 1.37 +/- 0.50 pg/ml, respectively; P = 0.47). The absence of an arteriovenous difference for bulk endothelin-1 across the lungs in the presence of tracer data indicating a substantial uptake implies that an amount of endothelin-1 quantitatively more or less equal to that removed is produced by the lung. The shapes of the dilution curves suggest that the tracer endothelin uptake by the lung is a one-way process without vascular reentry of tracer. We conclude that the dog lung is an important site for both uptake and release of endothelin-1.