Groups of 10 male and 10 female B6C3F1 mice were exposed to 0, 2, 4, 10, 20, or 40 ppm of formaldehyde vapor 6 h/day, 5 days/week for 13 weeks. Clinical abnormalities (dyspnea, listlessness, and hunched posture), significant mortality, and body weight loss were observed in the 40 ppm groups. Pathologic changes were observed in the nose, larynx, trachea, and bronchi of treated males and females and in the uterus and ovaries of treated females. Squamous metaplasia and inflammation were present in the nasal tissues of male and female mice in the 10, 20, and 40 ppm groups and in the larynx of males and females in the 20 and 40 ppm groups. The trachea had squamous metaplasia and hyperplasia of the epithelium in addition to submucosal fibrosis and inflammation in the 20 and 40 ppm groups. In some mice, epithelial-lined, irregular connective tissue bands spanned the tracheal lumen. Metaplasia of the bronchial epithelium was confined to the 40 ppm exposure groups. These effects on the respiratory system were more prevalent in male than in female mice. Hypoplasia of the uterus and ovaries, probably secondary to body weight loss, was confined to the 40 ppm exposure group. In conclusion, 13-week inhalation exposures of B6C3F1 mice to 10, 20, and 40 ppm of formaldehyde vapor induced histologic lesions in the upper respiratory system and concentrations of 40 ppm were lethal to those mice.