The metabolism of zeatin and that of 6-benzylaminopurine (BAP) have been compared in oat leaf segments in relation to the markedly differing ability of these cytokinins to retard senescence of such segments. Free BAP and a highly active senescence-retarding metabolite of BAP were detected in oat leaf segments supplied with BAP. The metabolite was identified by mass spectrometry and chromatography as 3-β-D-glucopyranosyl-BAP. The major metabolite of BAP was the 9-glucoside, but this lacked significant senescence-retarding activity. In contrast, in leaf segments supplied with zeatin, no free zeatin and no senescence-retarding metabolite of zeatin were detectable. The major metabolites of zeatin were adenosine, adenine nucleotides, the 9-glucoside, and unidentified polar metabolites. The differing activities of zeatin and BAP in the oat-leaf senescence bioassay appear to be, at least partially, a consequence of their differing metabolism and are not attributable to differences in uptake.