In vertebrates, endogenous thymidine kinase (TK) gene expression is strictly growth-dependent. Here we report that in continuously cycling Ltk-mouse fibroblasts, stably transfected with a vector expressing human TK cDNA from a constitutive promoter, enzyme activity rises 8-fold at the G1/S phase transition and declines again in G2. The mechanism did not involve changes in protein stability. When hTK was put under the control of a hormone-inducible promoter, production of high mRNA levels following addition of dexamethasone did not result in any enzyme activity in resting NIH-3T3tk- cells. After growth stimulation with serum, TK activity rose together with the onset of DNA synthesis only in the simultaneous presence of the hormone.