Absorption, tissue distribution, excretion, protein binding, and the metabolism of 14C labelled F-leurosine were studied in the rat. A triphasic curve for the disappearance of the drug from blood was found. The bile was the major route of excretion: 80% of radioactivity was recovered in the bile during the first 28 h after i.v. administration. 54% of the F-leurosine binds, in the concentration range of 0.1 mumol/1-0.5 mmol/l to plasma proteins. The TLC and HPLC data suggested that the bile of rats administered [14C]-F-leurosine contained either none or only small amount of metabolic products, which were most probably due to chemical decomposition rather then metabolism. Rat liver homogenates did not metabolize [14C] F-leurosine to any detectable extent.