A procedure is described for carrying out repetitive bronchopulmonary lavage in the rat, in which a given volume of iavage fluid is introduced into the lungs from a reservoir and then withdrawn from the lungs back into the reservoir, the process being repeated a number of times. During this procedure there is a nett release of endogenous surfactant and macrophages from the lungs. [14C] pulmonary surfactant was prepared from rats previously injected intravenously with [1‐14C] palmitate, and pulmonary macrophages labelled with 85Sr were prepared from rats which had received by intratracheal injection a suspension of fused clay particles labelled with 85Sr. It was shown by carrying out repetitive bronchopulmonary lavage with 0.15M‐NaCl containing either exogenous [14C]‐surfactant or [85Sr]‐macrophages that the release of endogenous surfactant and macrophages from the lung into the lavage fluid occurred concomitantly with the retention within the lung of radioactive exogenous surfactant and macrophages from the lavage fluid. It is concluded that the process of surfactant and macrophage detachment during bronchopulmonary lavage is reversible and that the exchange processes are of substantial magnitude.