The number of leucocytes and level of endogenous LTC4 in the pulp tissue were measured by a histological method and radioimmunoassay, respectively. When the mandibular incisor pulp was irritated by drilling a hole in the dentine without using any coolant, the number of polymorphonuclear leucocytes and lymphocytes and the concentration of LTC4 increased to 3.2, 1.9 and 1.8 times their respective levels in normal pulp 6 h after the injury. The total leucocyte number in blood collected from these rats was also increased significantly. In contrast, when the cavities cut in dentine were filled with a zinc oxide-eugenol mixture (powder: liquid 100 mg/25 microliters), the increase in the number of cells was significantly curtailed, and the LTC4 level fell to 50% of that in normal pulp within 1 h after the filling. No decrease in the LTC4 level was observed after filling with a zinc oxide-water mixture, but the level decreased in response to an increase in eugenol content in the zinc oxide-eugenol placed in the cavity. Biosynthesis of [14C]-HETE and HPETE from [14C]-arachidonic acid was inhibited by the addition of 10 microM eugenol to the pulp homogenate. Thus eugenol released from zinc oxide-eugenol inhibited the biosynthesis of lipoxygenase products and the early chemotactic accumulation of leucocytes.