The cytokinins in certain fractions prepared from extracts of immature sweet-corn (Zea mays L.) kernels using polystyrene ion-exchange resins have been further investigated. Cytokinins active in the radish cotyledon bioassay were purified from these fractions and identified as 9-β-D-glucopyranosylzeatin, 9-β-D-glucopyranosyldihydrozeatin, O-β-D-glucopyranosylzeatin. and O-β-D-glucopyranosyl-9-β-D-ribofuranosylzeatin. In addition, compounds which resemble zeatin and its glycosides in chromatographic behaviour and in ultraviolet absorption characteristics were purified from extracts of the same material by high-performance liquid chromatography. In addition to zeatin and zeatin riboside, the following compounds were identified unambiguously: O-β-D-glucopyranosyl-9-β-D-ribofuranosyldihydrozeatin, O-β-D-glucopyranosyldihydrozeatin, and hihydrozeatin riboside. A further compound was tentatively identified as O-β-D-glucopyranosylzeatin, and at least two unidentified compounds appeared to be new derivatives of zeatin. In identifying the above compounds, chemical-ionization mass spectrometry proved to be an invaluable complementary technique, yielding spectra showing intense protonated-molecular-ion peaks and also prominent structure-related fragmentation that was either not evident or very minor in the electron-impact spectra. An assessment of the relative importance of the various possible mechanisms for cytokinin modification and inactivation in mature sweet-corn kernels was made by supplying [(3)H]zeatin and [(3)H]zeatin riboside to such kernels after excision. The principal metabolites of zeatin were adenine nucleotides, adenosine and adenine, while little of the metabolite radioactivity was attributable to known O-glucosides. Adenine nucleotides and adenine were the principal metabolites of zeatin riboside, while lesser metabolites were identified as adenosine, dihydrozeatin, and the O-glucosides of dihydrozeatin and dihydrozeatin riboside. Side-chain cleavage, rather than side-chain modification, appears to be the dominant form of cytokinin metabolism in mature sweet-corn kernels.