Acetaldehyde or formaldehyde had a small effect on the number of pulmonary adenomas in NNK-treated A/J mice. Groups of 15 mice were treated with 0, 2.5, or 7.5 μmol NNK (i.p. in saline) in the presence or absence of 3 h of inhaled formaldehyde (16.5 ± 6.5 ppmv) or acetaldehyde (363 ± 7 ppmv). After 16 weeks, the mice were sacrificed and lung tumors were counted. *Significantly different from air alone, p < 0.01. **Significantly different from air alone, p < 0.05.