Creep, the progressive increase in pulmonary volume occurring under constant intratracheal pressure, has been studied in paralyzed dogs. Three types of variations of the initial rapid phase of creep are described: first, a reversible increase of creep, originating in the lungs, observed after a prolonged period of quiet ventilation and disappearing following forced inflations; second, a change of creep with the degree of lung inflation, shared by lungs and chest wall; and third, smaller variations occurring when the time between the successive inflations is shortened. The course of the late phase of creep is modified similarly by forced inflations. The analogies between the variations of creep related to previous volume history and the phenomenon of adaptation of the pressure-volume curve are stressed. While the most pronounced changes of creep connected with adaptation and with the degree of inflation can be attributed respectively to the opening of collapsed alveoli and to a time-dependent property of the alveolar surface film, the minor variations are possibly related to a tissue property of the lungs or to changes in intrathoracic blood volume.