Toxicology and carcinogenesis studies were conducted by exposing groups of 60 male and female F344/N rats and B6C3F1 mice to atmospheres containing 0, 0.01, 0.05, or 0.2 ppm hexachlorocyclopentadiene (approximately 98% pure) for 2 years. Under the conditions of these 2-year studies, there was no evidence of carcinogenic activity of hexachlorocyclopentadiene in male or female F344/N rats or B6C3F1 mice exposed to 0.01, 0.05, or 0.2 ppm. Exposure of rats to hexachlorocyclopentadiene produced pigmentation of the respiratory epithelium of the nose, trachea (males), and bronchi and bronchioles of the lung. Squamous metaplasia of the laryngeal epithelium occurred in female rats exposed to hexachlorocyclopentadiene. Suppurative inflammation of the nose as well as pigmentation of the respiratory mucosal epithelium occurred in mice exposed to hexachlorocyclopentadiene.