Abstract: 10-Hydroxy-△²-decenoic acid (10-HDA) is the main fatty acid extracted from the royal jelly of the honey bee. ³H-10HDA given intragastrically to mice and rats was rapidly absorbed. The plasma radioactivity reached maximum level in about 1 h.Radioactivity was rapidly taken up by various organs and has a strong affinity for tissues. The highest radioactivity leval was found in the liver(6 times as much as in plasma), followed by kidney, pancreas, fat, brain, spleen, heart and lung. At all time intervals, the radioactivity in various organs was higher than the plasma level. Peak radioactivity level in organs appeared 1 d after intragastric administration. The radioactivity was shown to pass the blood-cerebral barrier and those in organs disappeared slowly.In 31 d, the cumulative excretion of radioactivity was 85.4% of the total dose in urine and 13.5% in feces. The radioactivity excreted in bile in 24 h was 2.6%.The radioactivity in urine and bile measured by TLC autoradiography and liquid scintillation counting indicated that ³H-10HDA excreted in the urine and bile was mainly in unchanged form and nearly no metabolite was found in the urine and bile.After intragastric and intravenous administration of ³H-10HDA to rats and mice, the declie of radioactivity in the plasma was shown to be biphasie (a distribution phase and a elimination phase). The ³H-10HDA in plasma was found to be 63% bound to plasma protein. The main pharmacokinetic parameters calculated from the plasma radioactivity-time data are shown in table 1.