The metabolism of adenine and guanine, relating to the biosynthesis of caffeine, in excised shoot tips of tea was studied with micromolar amounts of adenine-[8-14C]or guanine-[8-14C]. Among the presumed precursors of caffeine biosynthesis, adenine was the most effective, whereas guanine was the least effective. After administration of a 'pulse' of adenine-[8-14C], almost all of the adenine-[14C]supplied disappeared by 30 hr, and 14C-labelled caffeine and RNA purine nucleotide (AMP and GMP)synthesis increased throughout the experimental period, whereas the radioactivities of free purine nucleotides, 7-methylxanthine and theobromine increased during the first 10 hr incubation period, followed by a steady decrease. By contrast, more than 45