D-penicillamine, previously shown to have a beneficial effect on radiation-induced pulmonary histopathology, was tested to determine its effect on function in the irradiated lung. Male rats were irradiated with 60Co gamma rays; half then received 10 mg D-penicillamine per day, and half received no further treatment. One to nine months after irradiation, animals were subjected to lung perfusion scans. Untreated irradiated rats exhibited hyperemia, hypoperfusion, and perfusion defects of the irradiated lung. In penicillamine-treated rats, the appearance of perfusion defects was delayed, the peak incidence and severity of the defects was reduced, and recovery from pulmonary hypoperfusion was accelerated. Thus, using functional criteria, penicillamine appears to improve arterial perfusion and to ameliorate radiation injury in the rat lung.