The reductive dehalogenation of halothane leading to 2-chloro-1,1,1-trifluoroethane (CTE) and 2-chloro-1,1-difluoroethylene (CDE) has been investigated in vitro using at liver microsomes under anaerobic conditions. The stimulation of NADPH oxidation by halothane as well as the formation of the products were dependent upon cytochrome P450 as indicated by their CO and metyrapone inhibition. After replacement of NADPH by sodium dithionite as a reducing agent CDE was the only product of the enzymatic reaction. The product pattern was influenced by pretreatment with 3-methylcholanthrene, benzo(a)pyrene, phenobarbitone and Arochlor 1254 and by addition of anti-cytochrome P450-PB immunoglobulin. The CTE:CDE ratio was shifted by addition or inhibition of cytochrome b5 and by pH variation indicating a crucial role of the second electron donation to cytochrome P450 in determining the product pattern. The intermediate complex of cytochrome P450 with a Soret band at 470 nm formed with halothane in reduced liver microsomes was shown to decompose spontaneously to give CDE. Therefore we propose the 470 nm peak to represent a cytochrome P450 Fe3+----CHCl-CF3 carbanion complex. From these results a reaction pathway could be derived which includes radical and carbanion intermediates as reactive precursors of CTE and CDE, respectively.