Radiotherapy has been used to treat malignant tumor of head and neck as one of the major modality. In spite of marked advancements in radiotherapy, damage to the salivary glands after irradiation has been inevitable, so many patients have been suffered from xerostomia. A radioisotope study was done to investigate the effectiveness of dexamethasone on salivary flow rate followination. Forty patients who were diagnosed pathologically as having a malignancy from Feb. 1988 to Feb. 1989 were evaluated. The results were as follows : 1) In cases of 1,000cGy, 3,000cGy and 6,000-7,000cGy irradiation, the salivary flow rates were reduced to 46.9%, 37.5% and 22.7% respectively on the side of the lesion of the parotid gland. 2) In cases of 1,000cGy, 3,000cGy and 6,000-7,000cGy irradiation, the salivary flow rates were reduced to 61.1%, 43.6% and 23.5% respectively on the contralateral side of the irradiated parotid gland. 3) In cases of 1,000cGy, 3,000cGy and 6,000-7,000cGy irradiation in the group pretreated with dexamethasone, the salivary flow rates were reduced to 59.8%, 52.7% and 36.2% respectively on the side of the lesion of the parotid gland(p<0.05). 4) In cases of 1,000cGy, 3,000cGy and 6,000-7,000cGy irradiation in the group pretreated with dexamethasone, the salivary flow rates were reduced to 73.4%, 61.0% and 45.2% respectively on the contralateral side of the parotid gland(p<0.05). 5) Regardless of administration of dexamethasone, there was no statistical significance on the submandibular gland(p>0.05). According to the results obtained above, the salivary flow was inversely proportional to the dosage of irradiation, and the dexamethasone slowed the reduction of the salivary flow rate.