Developing rice seeds rapidly accumulated storage lipids between 5 and 12 d after flowering. The contents of palmitic, oleic, and linoleic acids increased throughout seed development, while the alpha-linolenic acid content remained low. The activity of acyl-CoA synthetase varied coincidentally during the period of lipid accumulation, and rice seeds had a sufficient capacity to supply acyl-CoA substrates for TAG synthesis. Acyl-CoA synthetase showed a broad specificity for native FA of rice seeds except for stearic acid, and pi electrons of a delta9-delta11 double bond in the C16-C18 acyl chains were required for its maximal activity.