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Using semi-quantitative reverse transcription-PCR, we studied the expression of genes encoding caffeine synthase (TCS1), inosine-5′-monophosphate dehydrogenase (TIDH), Sadenosyl- l-methionine synthase (sAMS), phenylalanine ammonia-lyase (PAL) and α-tubulin (Tua1) in young and mature leaves, stems and roots of 4-month-old tea seedlings and young and old tea tissue cultures. The amounts of transcripts of TCS1 were much higher in young leaves than in other parts of the plant. Expression of TIDH was greater in leaves than in other parts. Little difference in the amounts of transcripts of PAL, sAMS and Tua1 was found between various organs of tea seedlings. Larger amounts of transcripts of TCS1 and PAL were found in young callus tissues than in old tissues. These results support our conclusion deriving from previous enzymatic and metabolic studies that caffeine is synthesized mainly in young leaf …