Static lung volume-pressure hysteresis can, at least in part, be explained by (I) the opening (recruitment) of closed lung units during inflation from low end-expiratory pressures, (2) the subsequent uniform expansion of open lung units during continued inflation, (3) uniform contraction of open lung units with deflation, and (4) closing (derecruitment) of open lung units with continued deflation to low end-expiratory pressures. (See Frazer et al., Respiration Physiol. 43: 237:246, 1981 and 61: 277-288, 1985.) The model assumes that closing (derecruitment) of open lung units during deflation occurs over a range of normally distributed positive end-expiratory pressures and that opening (recruitment) of closed lung units during inflation occurs at much higher "opening" pressures which are also normally distributed. Excised Tween-rinsed rat lungs exhibit less static hysteresis during a maximal inflation/deflation' cycle than do untreated lungs. For partial deflations from the maximally inflated state, however, tween-rinsed rat lungs exhibit substantial hysteresis for end-expiratory pressures which produce little hysteresis in untreated lungs. Our recruitment/derecruitment model still fits this phenomenon if rinsing with Tween shifts the mean of the distribution of closing pressures by about + 10 cm H2O.