Increased degradation of lung elastin during MV-O2 is suppressed in elafin-expressing mice. Cumulative 24-h urinary excretion of desmosine, normalized to creatinine excretion, is increased in 6-day-old wild-type mice after MV-O2 for 24 h, compared with unventilated controls that breathed 40% O2 for 24 h. In contrast, urinary desmosine excretion was unchanged in response to MV-O2 in elafin-expressing mice when compared with unventilated controls. *Significant difference between groups, P < 0.05. Values are means and SD; n = 4–12/group.