The present work with transgenic poplar lines producing varying levels of trans -zeatin suggests the existence of a switching threshold for triggering ckx gene expression or suppressing cytokinin-induced auxin. Cytokinins have an important role in growth and developmental processes of plants. Transgenic plants with varying levels of cellular cytokinin are convenient tools for studying its role in morphogenetic as well as molecular responses. In this work, the transgenic lines producing either high level of cellular trans-zeatin (HX lines) or moderate level (MX lines) were compared with regard to their cytokinin oxidase activities and cellular auxin content. The HX lines showed typical cytokinin phenotypes including leafy shoots and spontaneous shoot formation on hormone free medium. In contrast, the MX lines did not show any striking phenotypes. However, in leaf disk culture on hormone free medium, they regenerated roots and subsequently formed shoots from the roots. Determination of cellular IAA content revealed a significant increase in the level in MX lines but not in HX lines. Of nine cytokinin oxidase genes (ckx) examined by qPCR, five were activated in HX lines but not in MX lines. Among them, ckx4 appeared to play a key role in maintaining cellular cytokinin level since it showed more than 1,000-fold increase in HX lines and in the leaf disks of untransformed control exposed to exogenous cytokinins. Although low level of cellular cytokinin did not induce the expression of ckx genes, it appeared to trigger cellular IAA biosynthesis.