To evaluate a system for preserving and transporting lungs before transplantation, we removed the left lungs of 37 dogs, flushed them with a hypertonic solution having an electrolyte composition resembling intracellular fluid, and immersed them at 4 degrees C. for 7 to 24 hours. Some lungs were maintained at exactly 4 degrees C. during transport by means of a mixture of solid and liquid l-hexadecene. The lungs were allografted into immunosuppressed dogs whose right pulmonary artery was immediately ligated. Twelve recipients (32 per cent) survived 5 days or more solely on the function of the preserved lung. Four survived 10, 19, 40, and 40 days with lungs that had been preserved for 7 to 21 hours. Survival of recipients of preserved lungs (5 +/- 2 days) was equivalent to that of 75 comparably immunosuppressed recipients of nonpreserved allografts (6 +/- 1 days). One group of 10 dogs receiving lungs flushed against outflow resistance survived 12 +/- 5 days. In recipients of preserved allografts, arterial oxygen tensions remained in the normal range up to 5 weeks after transplantation, and radiographic infiltrates in the transplant were no greater than those present in recipients of nonpreserved transplants. Thus lungs transported and preserved up to 21 hours can provide total pulmonary function after transplantation and can function at least as well as nonpreserved transplants. The effectiveness and simplicity of this method are such that it might be considered for use in man.