Cytokinins and auxins are important regulators of plant growth and development, but there is incomplete and conflicting evidence that auxins affect cytokinin metabolism and vice versa. We have investigated these interactions in Nicotiana tabacum L. by separate in planta manipulation of levels of the hormones followed by analysis of the induced changes in the metabolism of the other hormone. Cytokinin-overproducing plants (expressing the Agrobacterium tumefaciens ipt gene) had lower than wild-type levels of free IAA, and reduced rates of IAA synthesis and turnover, but there were no differences in the profiles of metabolites they produced from fed IAA. Similarly, auxin-overproducing plants (expressing the A. tumefaciens iaaM and iaaH genes), had lower levels of the major cytokinins than wild-type plants and lower cytokinin oxidase activity, but there were no differences in the profiles of metabolites they produced from fed cytokinins. The data demonstrate that cytokinin or auxin overproduction decreases the content of the other hormone, apparently by decreasing its rate of synthesis and/or transport, rather than by increasing rates of turnover or conjugation. Implications for the importance of cytokinin : auxin ratios in plant development are considered. et al. 1988). Further evidence for the hormones' mutual effects on one another has come from studies of crown gall tissues in which mutations in Agrobacterium genes involved in cytokinin or auxin synthesis, led to changes in concentrations of both auxins and cytokinins (Akiyoshi et al. 1983, Rudelsheim et al. 1987, Ishikawa et al. 1988, McGaw et al. 1988). There are also reports of direct auxinactivation of the cytokinin-catabolising enzyme, cytokinin oxidase (Hare and Van Staden 1994). The data so far amassed on the reciprocal effects of cytokinins on auxins, and auxins on cytokinins, however, is incomplete and conflicting. Our understanding of these hormonal interactions and their role in regulating plant development would therefore be greatly enhanced by separate in planta manipulation of the levels of the hormones, followed by high-resolution, kinetic analysis of the induced changes in the metabolism of the other hormone. In this investigation we have used fully-regenerated, fertile, seedsown, cytokinin and auxin overproducing, transgenic Nicotiana tabacum L. plants to study effects of cytokinin on auxin metabolism and vice versa. The data obtained has given a number of insights into the interactions, or cross-communication, between the two hormones.