Acetate-1-C14and acetate-2-C14were supplied to slices of developing castor bean endosperm. The molecules were extensively incorporated into long-chain fatty acids, water-soluble compounds, and protein. Oleic acid was the fatty acid initially labelled from acetate and it was the precursor of ricinoleic acid. Aerobic conditions were required for the formation of oleic acid and for the conversion of oleic acid to ricinoleic acid. Under anaerobic conditions the incorporation of acetate carbon into fatty acids was inhibited more than 90% and almost all of the C14was found in stearic and palmitic acids. Stearic acid appeared to be formed first and palmitic acid appeared to be derived from it through a shortening of the chain. The position of linoleic acid in the fatty acid interconversions was not clear except that it was not a free intermediate in the conversion of oleic acid to ricinoleic acid.Malonate-C14was only absorbed slightly by the tissue and although absorption could be increased by the use of diethyl malonate the metabolism of the compound was not facilitated. Because of its poor utilization by the tissue the role of malonate in long-chain fatty acid synthesis in this tissue could not be ascertained.