Expts. with a model system and with apneic anesthetized dogs show expiratory flow patterns which fit an equation, that states that flow (V) decreases with time (t) according to the curve t = k1/K loge V - (2k2/K)V plus a constant of integration. This was derived from the assumptions a) that the resistance, P/V, equals [k1 + k2 V], and b) that the elastance, P/V equals K. It was possible to obtain values of ki, k2, and K from the expiratory flow patterns. Inertia was found to be negligible. A method is detailed for using this type of data to evaluate hysteresis. In one dog, hysteresis was found to be less than one-third the value of static elastance. Normal unanesthetized human expirations do not fit this pattern. The implication is that they are not simply "passive".