Lypoxygenase activity of lymphocytes towards exogenous substrate linolic acid and the level of endogenous arachidonate metabolite 15-HETE are increased within 6 h and decreased below the control level at 24 h after 1 Gy X-ray irradiation of Wistar rats. DNA fragmentation is stimulated both in vivo within 12 h and during in vitro incubation of lymphocytes, obtained 3 h following irradiation. DNA analysis by agarose gel shows that DNA fragments are multiples of the 200 base pair unit. When 20 microM nordihydroguayaretic acid, a selective inhibitor of arachidonate lypooxidation, was added to the incubation medium, DNA fragmentation was observed to be significantly less, especially at the early steps of incubation.