Hexachlorocyclopentadiene (HEX) is an unsaturated, highly reactive, chlorinated cyclic hydrocarbon of low water solubility. HEX is a chemical intermediate used in the manufacture of chlorinated pesticides and flame retardants with no end uses of its own. The major source of environmental contamination by HEX is the aqueous discharge from production facilities, with small concentrations present as contaminants in commercial products made from it. HEX is not readily absorbed by epithelial tissues because it is highly reactive, especially with the contents of the GI tract. HEX is moderately toxic when given orally, but has been estimated to be 100 times more toxic when inhaled. The data base for the long-term toxicity of HEX is very limited. A chronic inhalation bioassay is being conducted by the NTP and may provide data regarding any carcinogenic potential of HEX.