Previous work with Chinese hamster cells suggests that thymidine kinase deficiency and loss of potential for plating in HAT medium may arise by a process of mutation coupled with site-specific repression by bromodeoxyuridine at the tk locus. In this study, tk- Chinese hamster cells were exposed to a series of inductors to determine whether revertants for the putative second stage originate by genetic or epigenetic change. Brief exposure to 5-azacytidine resulted in massive conversion to the HAT+ state, and revertants showed levels of thymidine kinase activity intermediate between those of tk- and wild-type cells. By contrast, incidence of HAT+ cells rose only slightly in populations mutagenized with ethyl methanesulfonate. Large increases in frequency of HAT+ cells were obtained by treatment with n-butyrate and L-ethionine, which affect gene expression in other cell systems but have no known mutagenic potential. Induction of HAT+ revertants seems to be mediated by a stable epigenetic shift, which reverses the gradual extinction of thymidine kinase activity in the parent cells. The data support the view that induction in Chinese hamster cells results from changes in DNA methylation patterns, and suggests studies to define the process in molecular terms.