U n i t 6 Associ6e au CNRS en Wveloppement Concert6 avec l'INSERn, UA 400, 45 rue des Saints Phres, 73270 P a r i s cedex 06, France. SUMMARY Synthesis and chromatographic separation o f (E) and (Z) isomers o f (ring-U-l4 C) hexanal phenylhydrazone 2 : (ring-U-' 4C) phenylhydrazine hydrochlor i d e 1 was synthesized from l a b e l l e d a n i l i n e hydrochloride, and p u r i f i c at i o n was accomplished by reverse phase low pressure l i q u i d chromatrography. The crude f r e e base o f 1 was condensed w i t h hexanal t o give a mixture o f (E) and ( 2 ) isomers o f 2. The conditions f o r t h e i r separation by reverse phase HPLC were optimized on an a n a l y t i c a l column and then used for the preparative work which was the most e f f e c t i v e and fastest method. Structure and configuration o f (E) and (Z) isomers o f (ring-U-14C) hexan a l phenylhydrazone (SA = 13.3 mCi/mmol) were proven by ' H-NMR and mass spectroscopy.