The effect of a series of trihalomethanes, CHCl3, CHBrCl2, CHBr2Cl, and CHBr3, on in vitro lipid metabolism was compared using rat liver slices. The incorporation of 32Pi and [3H]glycerol into phospholipid of liver slices was inhibited in the presence of trihalomethanes. The inhibitory effect followed the number of bromine atoms in the trihalomethane molecule in the initial period of incubation. CHBr3 markedly inhibited the incorporation of [3H]glycerol into triacylglycerol (TG); CHBr2Cl was less effective and the other two trihalomethanes were without effect. The activities of glycerophosphate acyltransferase, phosphatidate phosphatase and diacylglycerol acyltransferase were changed by the exposure to trihalomethanes. The effects of CHBr2Cl and CHBr3 were much more severe as compared to those of CHBrCl2 and CHCl3. The change in the enzyme activities could explain the alteration in the incorporation of [3H]glycerol into TG.