Increased fluid intake (polydipsia) is one of the classic symptoms of diabetes mellitus. Xerostomia (dry mouth) and resultant thirst are other symptoms of the disease and bear a close relationship to polydipsia. The xerostomia in individuals with diabetes is primarily due to decreased saliva flow which appears to be associated with degenerative changes in the salivary glands. This study examines the response of the rat submandibular gland to streptozotocin induced diabetes mellitus. Adult male rats were given a single I.V. dose of streptozotocin (65 mg/kg body weight) in citrate buffer (pH 4.5). Salivary glands were examined by light and electron microscopy at 4, 8 and 24 h and 3, 7, 14 and 21 days posttreatment. The changes in the acinar cells were characterized by an accumulation of secretory material within the cytoplasm. This secretory protein accumulation was followed by degenerative changes in the acinar cells which frequently resulted in cell death and replacement of secretory cells by connective tissue elements. The loss of secretory volume and potential changes in secretory kinetics are discussed with regard to the xerostomia, thirst and polydipsia exhibited by individuals with diabetes mellitus.