The effect of ulinastatin on delayed neuronal death was studied in the gerbil. The animals were divided into 2 groups according to the temperatures of the temporalis muscle, tympanic membrane, and the rectum. One group consisted of 36 animals in whom the temperature was maintained at 36.5 degrees C and the another was at 38 degrees C. They were anesthetized with isoflurane and injected intravenously with 5 ml. kg-1 of normal saline solution, 100 mg. kg-1 of alpha-tocopherol, 250,000 units. kg-1 of ulinastatin, or 500,000 units. kg-1 of ulinastatin, respectively. Transient ischemia was then induced by occluding bilateral common carotid arteries for 5 min. Six days later, they were sacrificed, and the brain tissues were fixed and stained for histopathological analysis of hippocampal CA1 regions. alpha-Tocopherol and ulinastatin prevented delayed neuronal death in the 36.5 degrees C group, but did not in the 38 degrees C group. alpha-tocopherol is recognized as an intrinsic radical scavenger, and known to prevent delayed neuronal death. Compared with 100 mg. kg-1 of alpha-tocopherol, 500,000 units. kg-1 of ulinastatin had the same protective effect on delayed neuronal death at 36.5 degrees C. We concluded that administration of ulinastatin prevented delayed neuronal death at 36.5 degrees C.