We present the results of a study of the effects of chronic exposure to elevated ozone on the cytokinins of mature beech trees. Methods for analysing the cytokinin (CK) content of beech (FAGUS SYLVATICA) were developed using seven enzyme-linked immunosorbent assays (ELISAs). Samples taken during 2003 and 2004 from 10 mature beech trees in Kranzberg forest, 5 trees exposed to twice ambient ozone (2 x O(3)) by free-air fumigation and 5 control trees (1 x O(3)), were analysed. In 2003 and 2004 the cytokinin content of leaf samples followed a similar seasonal pattern. In leaf samples, the content of aromatic types was equal to that of the isoprenoid types. In root samples, the level of aromatic types was no different from leaves, but that of the isoprenoid types was much higher. Leaf and phloem cytokinin contents for 2 x O(3) trees were lower than for 1 x O(3) at almost all sampling times. The effect of ozone was greater for leaves in the sun crown than for leaves in the shade crown. By contrast, the root and xylem contents of cytokinin for 2 x O(3) trees were greatly elevated over the values for 1 x O(3) trees early in the growing season. We propose that O(3)-associated CK destruction in leaves reduces CK-mediated root growth suppression. The resulting increases in root growth and ectomycorrhiza, reported by other groups in the Kranzberg forest project, are likely to be responsible for the increased CK export in xylem, although O(3)-associated CK destruction in the leaves appears to nullify this increase.