Endosperm of Sechium edule seed was incubated with [U-14C]adenine and [13C]mevalonic acid for 17 h. Following incubation, cytokinins were extracted and purified by HPLC. Using GC-MS, incorporation of [13C]MVA into isopentenyladenine and zeatin riboside was demonstrated. This is the first unequivocal demonstration of cytokinin biosynthesis in a normal plant tissue because precursors incorporation into cytokinins was demonstrated by MS. The significance of these results is also discussed in relation to a possible mononucleotide-independent pathway of cytokinin biosynthesis.