The utilization of fatty acids from the culture medium has been examined in preimplantation mouse embryos developing in vitro. Incorporation of exogenous fatty acid into embryo lipids was examined by culturing 8-cell mouse embryos for 2h in a medium containing 0.1 mM [9, 10-3H]palmitic acid (900 mCi/mmol). Lipids were extracted from the embryos, and the total lipid extract was fractionated into various neutral lipid and polar lipid classes by thin-layer chromatography. Most of the radioactivity, over 93%, was recovered in neutral glycerides (mono- di-, and triacylglycerols). About 2% of the total radioactivity was recovered in other neutral lipid species including fatty acids, fatty alcohols, and sterol esters. The remainder of the radioactivity was recovered in polar lipids. Seventy-four per cent of the polar lipid radioactivity was present in the amine phosphatides, inositol and/or serine phosphatides, sphingomyelin, choline lysophosphatides, sulfatides, cerebrosides, and monoglycosylglycerides. Chemical degradation studies of labelled embryo lipids indicated that the tritium lable was entering into embryo lipids as the fatty acid and not via metabolic recycling. The oxidation of exogenous fatty acids by mouse embryos was assessed by incubating variously staged embryos for 4h in medium containing 0.1 mM [U-14C]palmitic acid (50 mCi/mmol) and quantitating the production of 14CO2. The rate of fatty acid oxidation was found to be relatively constant from the unfertilized egg up to the 8-cell stage and then increase significantly between the 8-cell and late blastocyst stages. The results suggest that preimplantation mouse embryos developing in vitro can utilize fatty acids from the medium both for incorporation into embryo lipids and for energy production via oxidation.