The absorption enhancer, didecanoylphosphatidylcholine (DDPC), improves the nasal absorption of human growth hormone in rabbits. We elucidated the uptake and the metabolism of 1,2-di[1-C]decanoyl-L-3-phosphatidylcholine and 1,2-didecanoyl-L-3-phosphatidyl[N-methyl-H]choline in rabbit nasal mucosa in vivo. One minute after nasal application of DDPC, 4.4-7.5% of the applied dose was found absorbed into the mucosa. The retained radioactivity left the tissue in