The human c-myc gene was introduced and transiently expressed in adult rat hepatocyte cultures by the technique of electroporation and its effect on DNA synthesis was examined. Epidermal growth factor (EGF) has been found to stimulate a wave of DNA synthesis in electroporated rat hepatocytes. Hepatocyte cultures electroporated with the c-myc gene showed a potentiation of this EGF effect exhibiting rates of DNA synthesis up to 50% greater than those of control electroporated cultures, as determined by [3H]thymidine labeling of cell nuclei. This potentiation was dependent on the amount of c-myc DNA transfected. The potentiation was due neither to an alteration in the dose-response of the stimulatory effect of EGF nor to a change in the time course of the DNA synthesis wave.