The nature and origin of the expiratory activity patterns were investigated by recording the expiratory action potentials of the thyro-arytenoid muscle of the dog during changing vagal activity controlled by artificial inflation of the lungs. Intensity of contraction varied with the degree of pulmonary inflation. When the lungs were inflated in a progressive manner the synchronous expiratory contraction was of the slowly augmenting type. When an initially high but slowly waning vagal discharge occurred in phase with the expiratory discharge a rapidly augmenting and slowly waning contraction was obtained. The waning vagal activity occurring during normal expiration is evidently an extremely important factor in the production of the rapidly augmenting and slowly waning expiratory contraction. Architectural arrangements of the inspiratory and expiratory half-centers underlying the slowly augmenting and steady state contractions are discussed. Since vagal activity evokes a purely reflex activity as well as a reflex modification of a prevailing expiratory discharge, the rapidly augmenting and slowly waning expiratory activity may be primarily a reflex response or a reflex modification of the steady state discharge.