Lung volume increase after cigarette smoke exposure is not reversible after smoking cessation. The relative lung volume was measured by fluid displacement. The relative lung volumes were determined after 20 weeks air exposure (white bar), after 20 weeks smoke exposure (black bar) and after 20 weeks smoke exposure plus a smoking cessation period of 8 weeks (grey bar). n = 4-5 animals per group. Values are expressed as mean +/- S.E.M. *P ≤ 0.05; significantly different from the control group.