Several attempts have been made to describe the relation that exists between tobacco smoke and emphysema, through different experimental models of the active smoker. Despite the negative effects that involuntarily inhaled tobacco smoke can have on the lung, no experimental model of the passive smoker has been proposed. In this study, an experimental model of the involuntary smoker is described and the following hypothesis proposed: Passive exposure to tobacco smoke produces morphological alterations in the rat lung, which are compatible with emphysema. Emphysema will be considered to have been caused when enlargement of the distal airspaces of the lung and lung tissue destruction are demonstrated. Sixty Wistar rats were used, divided into two groups: a control group and a group that was passively exposed to tobacco smoke for a period of 3 months. A morphometrical study of the lung was performed using a computerized system. To demonstrate enlargement of the distal airspaces of the lung, the following variables were quantified: alveolar chord and mean linear intercept index (Lm); tissue loss was demonstrated by means of the quantification of the variables: tissue density, internal alveolar perimeter (IAP), and wall thickness (WT). The elastic fiber was also quantified. The animals that were exposed to tobacco smoke displayed the following significant alterations (p < .05): an increase of the alveolar chord and of the Lm, a decrease of the IAP, WT, and tissue density, and a loss of the elastic component of the lung. On the basis of these findings, it is concluded that the rats that were exposed in a passive way to tobacco smoke display morphological pulmonary alterations that are compatible with the definition of emphysema.