Reaction of an homogenate of the rat nasal mucosa with formaldehyde (CH2O) followed by solubilization and extraction in a strongly denaturing aqueous-immiscible organic solvent mixture decreased the quantity of nucleic acids that remained in aqueous solution in comparison with a control homogenate untreated with CH2O. The absent DNA and RNA were located in the interface between the aqueous and organic phases, from which they could be recovered only after enzymatic proteolysis. It is concluded that interfacial nucleic acids were cross-linked to proteins by CH2O. The concentration of cross-links was estimated with 14CH2O; under conditions that rendered 48% of the DNA nonextractable from proteins, there was less than one cross-link per 28,000 nucleotide residues. Exposure of rats to airborne CH2O at concentrations of 0, 2, 6, 15, and 30 ppm (6 hr/day for 2 days) resulted in a statistically significant increase in the percentage of the total DNA from the respiratory mucosa that was located in the interface at concentrations of 6 ppm and higher. However, the percentage of DNA from the olfactory mucosa located in the interface was not increased by CH2O exposure. Analysis of nasal mucosal DNA by ultracentrifugation in CsCl density gradients provided no evidence of a change in the buoyant density of the DNA caused by reaction with CH2O. The results indicate that CH2O may induce DNA-protein cross-links in the respiratory but not in the olfactory mucosa at concentrations equal to or greater than 6 ppm.