We developed a suicide gene therapy protocol for the treatment of brain tumors. This protocol is based on the expression of the thymidine kinase gene of Herpes simplex virus: this protein is toxic for dividing cells in the presence of specific drugs such as acyclovir or gancyclovir. We developed an adenoviral vector to transfer and express the viral thymidine kinase into experimental C6 tumors implanted in Wistar rat brain. Rats were then treated with injection of gancyclovir and we observed significant reduction in tumor growth and an increase in survival for the treated rats as compared to control animals (injected with the corresponding buffer). Furthermore, no major side effects were noticed.