Abstract 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% of the guanine-[8-14C] supplied remained unchanged even after a 120 hr period. The main products of guanine-[8-14C] metabolism in tea shoot tips were guanine nucleotides, theobromine, caffeine and the GMP of RNA. The results support the hypothesis that the purine nucleotides are synthesized from adenine and guanine via the pathway of purine salvage. Adenylate is readily converted into other purine nucleotides, whereas the conversion rate of guanylate into other purine nucleotides is very low. The results also support the view that 7-methylxanthine and theobromine are precursors of caffeine. For the origin of the purine ring in caffeine, purine nucleotides in the nucleotide pool rather than in nucleic acids are suggested.