Excised immature radish cotyledons were allowed to take up [3H]6-benzylaminopurine for up to 5 h and were then transferred to cytokinin.free medium for various intervals, after which they were extracted. The cytokinin was rapidly metabolized by the tissue to two main products which accounted for 20 per cent of the extracted radioactivity after 1 h of uptake, and for 95 per cent after transfer to the cytokinin-free medium. These metabolites were coincident with cytokinin activity on chromatograms of the cotyledon extract and were also formed when 6-benzylaminopurine was supplied to 9-d-old derooted seedlings through the transpiration stream. The metabolites were identified by mass spectrometry, u.v. spectro scopy and chemical degradation as 6-benzylarnino-7-glucosylpurine and 6-benzylamino-9. glucosylpurine. Several minor metabolites were detected in the derooted seedlings; one was 6-benzylaminopurine riboside while another appeared to possess high cytokinin activity.