Fluroxene is highly toxic to several animal species. This toxicity is enhanced by induction of raised levels of hepatic microsomal enzymes. Experiments in rats are described which seek to assess the rleative contribution to this toxicity of the individual component groups of the fluroxene molecule. Though results point to the trifluoroethyl moiety of fluroxene as that aspect of the molecule most responsible for the observed mortality, reduction of the vinyl group modifies the pattern of liver injury. That the liver necrosis, manifest following fluroxene anesthesia in the presence of microsomal induction, is alone the direct cause of the acute death of experimental animals is questioned.