We have measured the instantaneous extraction of prostaglandin E1 (PGE1) after a single bolus injection of PGE1 and indocyanine green dye into the pulmonary artery of isolated cat lungs. The extraction ratio versus time curves exhibited a characteristic shape; at early times they were concave upward and later in time concave downward. To evaluate this date we utilized a model in which themechanism of PGE1 uptake is saturable and follows Michaelis-Menten kinetics. The model assumes heterogeneous perfusion of the exchanging region of the lung and can be solved by regression analysis to obtain Km and Vmax from the data collected from a single bolus injection. The results indicate that the shape of the extraction ratio curve manifests the influence of nonlinear uptake and heterogeneous perfusion and that the kinetic parameters may be calculated from the data obtained after a single bolus injection ofPGE1.