Experimental evidence suggests that the lung behaves as if it is composed of a large population of units which are recruited and derecruited during lung expansion and contraction. This study combines two previous models in order to estimate the probability distribution function describing lung unit opening pressures and the resulting alveolar surface area-volume relationship of the excised rat lung during inflation. Results indicate that the opening pressures of lung units during inflation can be described by a normal distribution. The end-expiratory pressure (EEP) has a large effect on the number of lung units that open during inflation and the properties of the area-volume relationship of the lung, but the distribution of opening pressures of individual lung units is fairly consistent regardless of EEP. This study also presents evidence that when the normalized lung area-volume relationship is represented by the equation [A(L)]N = [phiV(L)]N(n) during inflation from the closed state, the expansion coefficient n is between 0.86 and 1. This result supports the theory that, for inflation from EEPs below 4 cmH2O, lung expansion occurs in part by the recruitment of lung units and not solely by the expansion of open units.