The ipt gene of Agrobacterium tumefaciens T-DNA encodes an isopentenyltransferase which causes cytokinin overproduction and developmental alterations in transformed plants. A chimaeric gene constructed by positioning the ipt coding region under the control of the hsp70 gene from Drosophila melanogaster allows heat-regulated expression in transgenic plant tissue. Heat-shock treatment of tobacco calli transgenic for the chimaeric hsipt gene increases the endogenous cytokinin concentration and enables these calli to grow on cytokinin-free medium. Transgenic plants regenerated from calli transformed with the hsipt gene and grown at normal temperature are phenotypically normal.