Secondary cerebral edema is no less significant than direct injury of the brain parenchyma as a factor to markedly aggravate the prognosis of cerebrovascular disturbance. Accordingly, theorapeutic measures for cerebral edema are very important.For the purpose of studying the anti-cerebral edema effect of dexamethasone, we had cerebral hemorrhage induced experimentally in rabbit and studied the occurrence of cerebral edema, effect of dexamethasone administration on it and the method of administration. Experimental animals used were 197 male rabbits weighing 2.4-2.6 kg about 12 weeks after birth.Experimental lesion of cerebral hemorrhage was prepared by the cold injury method according to Klatzo's method.In order to examine the course of cerebral edema in the acute phase of cerebral hemorrhage, the animals were decapitated on a time-course basis after cold injury ; the brain was taken out and divided into the injured part and intact part ; the brain was dried with a vacuum freezedrier ; the brain weight before and after drying was measured with a chemical balance and the water content of the brain was calculated.The RISA transfer rate was also measured on a tune course basis so as to find the conditions of the blood-brain barrier of the impaired brain which plays an important role in the occurrence of cerebral edema.After cold injury, dexamethasone in doses of 0.4 mg/kg/day and 2 mg/kgiday was administered for three days ; the water content of the brain and RISA transfer rate were measured, and a comparative study was made of the administration group and the non-administration group. The period of administration was set at 1 day, 2 days, 3 days, 4 days and 7 days. On these five groups of administration, measurement was made of the water content of the brain one day after and three days after the final day of administration, results of which are as follows.1) Experimentally, the cerebral edema observable in the acute phase of cerebral hemorrhage began to appear six hours after the onset of the disease and reached a peak 12 to 48 hours after.2) A large-dose administration of dexamethasone had an inhibitory effect on cerebral edema incidental to cerebral hemorrhage. Administration for three to four days was free of rebound phenomenon and appeared to be appropriate as the period of administration.