Purpose In our study, after applying a single dose of 612 cGy irradiation, we aimed to observe the role of free radicals on tissue damage in liver caused by irradiation, by measuring the nitric oxide (NO) level, superoxide dismutase (SOD) and glutathione peroxidase (GPx) enzyme activities, and the amount of thiobarbituric acid reactive substance (TBARS), which is an indicator of free-radical damage. On the other hand we investigated whether tissue damage can be prevented by vitamin A or not. Materials and methods The study was performed on three groups: (1) the control group, (2) the group to which irradiation was administrated, and (3) the group which was given irradiation + vitamin A. The irradiation group of animals was given a single dose of gamma irradiation at a sublethal dose. In the group to which both irradiation and vitamin A were administered, vitamin A was given for 2 days prior to irradiation. The amount of NO was measured by electron spin resonance (ESR) spectroscopy, whereas SOD, GPx and TBARS were measured by spectrophotometry. Results and conclusion As a result of irradiation-mediated tissue damage in liver, we observed a NO loss and an increase in TBARS amount. Administration of vitamin A before irradiation resulted in an increase in both NO and TBARS and a decrease in SOD and GPx enzyme activities. Together, these data indicate that vitamin A may play an exacerbative role in free-radical-mediated tissue damage.