Abstract: In the present paper, the absorption, distribution and excretion of ³H-harringtonine in normal and tumor-bearing rats and mice are reported. After ³H-harringtonine was injected intravenously to normal rats, blood radioactivity level decreased rapidly with two biological phases of half-life, the first one (fast) being about 3.5 minutes and the other (slow) 50 minutes. Fifteen minutes after intravenous injection of ³H-harringtonine to normal rats, the highest level of radioactivity was present in the kidneys, while appreciable radioactivity was present in the liver, bone marrow, lung, heart, gastrointestinal tract, spleen and muscle. The radioactivity in the testis, whole blood and brain was low. Two hours after injection, the drug concentration in various tissues decreased rapidly, but that of the bone marrow decreased more slowly, thus forming the highest concentration among the tissues tested. Twenty-four hours after administration of ³H-harringtonine, only very low levels of activity were found in all tissues. In tumor-bearing mice a similar distribution pattern was observed. Twenty-four hours after intravenous injection of ³H-harringtonine to normal rats, the total urinary excretion of radioactivity was found to be about 30.2%, while about 16.6% was present in the feces. It was also shown that approximately 14.5% of the administered radioactivity was excreted as unchanged harringtonine within 24-hour period. Biliary excretion was also an important route of excretion. Normal rats excreted 24.5% of the administered radioactivity in the bile in 24 hours, while about 17.1% was excreted as unchanged harringtonine. When ³H-harringtonine was administered orally to normal mice, it was absorbed rapidly but incompletely.