The fate of carbon tetrachloride and hexachloroethane was studied in sheep, rabbits, cockerels, ducks and liver flukes. A new method was described for detection and estimation of hexachloroethane and its metabolites pentachloroethane and tetrachloroethylene in biological extracts using a multicolumn gas-liquid chromatographic technique. Pentachloroethane had not prieviously been reported as a metabolite of hexachloroethane and was itself metabolised to tetrachloroethylene. A new metabolite of carbon tetrachloride, hexachloroethane, was described and may have arisen by dimerisation of free trichloromethyl radicals. Detection of the dimer of such radicals may provide evidence for the formation of trichloromethyl radicals from carbon tetrachloride ; a current hypothesis suggests that the toxicity of the drug is due to liberation of these radicals in vivo. The biliary excretion of carbon tetrachloride and hexachloroethane in several species including sheep was studied. The effect of carbon tetrachloride, hexachloroethane, pentachloroethane, tetrachloroathylene, tetrachlorodifluoroethanes and bistrichloromethylbenzene on plasma enzyme activities in sheep was studied. Carbon tetrachloride and hexachloroethane were hepatotoxic in sheep, but probably not in cockerels and ducks. Hepatotoxicity due to hexachloroethane had not prieviously been described and was detected by elevation of plasma enzyme activity and reduced bromsulphthalein dye transfer rates. The hepatotoxicity of carbon tetrachloride for sheep and tolerance of cockerels and ducks to the drug was discussed. The in-vitro toxicity on liver flukes of various substances was studied. Liver flukes metabolised carbon tetrachloride and hexachloroethane in vitro. Carbon tetrachloride, carbon tetrachloride treated liver lipid and a carbon tetrachloridemethyl oleate complex were toxic to liver flukes in vitro, in the presence of sheep bile. The fasciocidal action of carbon tetrachloride was discussed and related to the toxicity of the drug in mammals.