The comparative narcotic effects of halogenated hydrocarbons were studied in mice. White mice were exposed to chloroform (67663), methylene-chloride (75092), tetrachloromethane (67663), tetrachloroethane (79345), dichloroethylene (25323302), ethylene-chloride (107062), ethylidene-chloride (75343), propyl-chloride (540545), ethyl-bromide (74964), and propyl-bromide (106945) in an 11 liter bottle. The animals were observed for 2 to 5 days. The efficacy of the compounds was determined by noting how soon the animals fell over and lay on their sides. All dead and surviving animals after chloroform exposure were necropsied. The relative effective strengths of the compounds as narcotic agents, based on a scale in which chloroform was assigned the value 1, were: propyl-chloride 0.14, ethyl-bromide 0.25, methylene-chloride 0.29, ethylidine-chloride 0.53, propyl-bromide 0.53, dichloroethylene 0.64, tetrachloromethane 0.66, ethylene-chloride 0.95, chloroform 1, and tetrachloroethane 3.5. Pathological changes induced by chloroform included fatty infiltration of the liver with subsequent fatty degeneration. Fatty degeneration in the kidneys and heart was noted. The author concludes that the narcotic effectiveness of the compounds increases in the order: propyl-chloride, ethyl-bromide, methylene-chloride, ethylidene-chloride, propyl-bromide, dichloroethylene, tetrachloromethane, ethylene-chloride, chloroform, and tetrachloroethane.