The stability of [(3)H]zeatin riboside supplied to freshly excised tobacco pith explants was found to be inversely related to α-naphthaleneacetic acid concentration in the incubation medium. At higher concentrations of α-naphthaleneacetic acid greater breakdown of [(3)H]zeatin riboside was indicated by higher levels of degradative metabolites (adenine, adenosine and adenosine nucleotides) formed. This auxin effect on cytokinin metabolism appears to be mediated, at least in part, through cytokinin oxidase. The results of in-vitro assays carried out with partially purified enzyme from corn kernels substantiale this conclusion. These findings are discussed in relation to recent observations of auxin and cytokinin levels in crown-gall tumours with altered morphology.