Male Syrian hamsters were exposed to 30 +/- 5 ppm nitrogen dioxide for 22 hr daily for 3 wk. Nitrogen dioxide-exposed hamsters sacrificed at various times during the 3 wk exposure showed a general loss of body weight and an increased dry lung weight when compared with the controls, which were housed in a similar, but nitrogen dioxide-free environment. Analysis of total lung collagen and total lung elastin revealed a net decrease in the moieties within 4 and 10 days, respectively, following commencement of nitrogen dioxide exposure. Total lung collagen returned toward pre-exposure levels by the 14th day of nitrogen dioxide exposure. Total lung elastin did not return toward normal until termination of nitrogen dioxide exposure. Recovery in room air for 3 wk following 21 days of nitrogen dioxide exposure restored the total pulmonary collagen and elastin to valutin and collagen degradation and synthesis differ during and after nitrogen dioxide exposure. Lung collagen loss was observed earlier and was restored to normal values during the continuation of nitrogen dioxide exposure. Lung elastin loss occurred later and persisted during the entire period of exposure but returned to normal after exposure was terminated.