AbstractDeuterium labelled analogues of the major zeatin derived cytokinins have been synthesized to serve as internal standards for mass spectrometric quantitation. A procedure for the isolation, high performance liquid chromatographic purification and direct probe mass spectrometric analysis of these cytokinins has been devised and applied to the analysis ofZea mayskernels where the levels of endogenous zeatin, zeatin‐9β‐riboside and zeatin‐9β‐glucoside were estimated by isotope dilution. Additionally, the deuterated compounds have been utilized to confirm the identity of thein vitroenzymic glucosylation products