Phenylalanine ammonia lyase (PAL) activity in the leaves of Eucalyptus sideroxylon and E. sieberi was determined at intervals throughout the year. Activity increased as leaves enlarged then declined as senescence approached. Some leaves lacked activity and there was variation within and particularly between seasons. The PAL species present was not affected by leaf polyphenols which lower the activity of the PAL species from barley. PAL activity was not detected in the zone between the sapwood and heartwood of E. polyanthemos and Pinus radiata. Pinosylvin synthase activity did not increase in the intermediate zone of P. radiata. Dehydroshikimate reductase could not be detected in leaves or woody tissues. Labelled phenylalanine was rapidly converted to polyphenols in E. sideroxylon leaves of low PAL activity but the sequence of polyphenols formed was different from that found when labelled glucose was fed. Evidence is presented supporting the view that neither phenylalanine nor its ammonia lyase were obligatory in the biosynthesis of eucalypt leaf or heartwood polyphenols.