Ceramide mediates apoptosis and is upregulated by oxidative stress. To reveal the causative agent of diabetes-induced complications, we examined the changes in ceramide metabolism during diabetes. Two and 8 weeks after intraperitoneal injection of streptozotocin (STZ: 40 mg kg⁻¹ body weight) to rats, tissue ceramide levels were analyzed by liquid chromatography-electrospray tandem mass spectrometry (LC-MS/MS). Blood glucose was significantly increased 2 weeks after STZ administration. Alanine aminotransferase (ALT), aspartate aminotransferase (AST), and blood urea nitrogen (BUN) levels were also increased in the diabetic rats, suggesting that hepatic and renal damage was induced by STZ administration. Vitamin C, an indicator of oxidative stress, was significantly decreased in the plasma, liver, and kidney of rats 2 weeks after STZ administration. Although no differences in hepatic ceramide levels were observed between the control and diabetic rats, plasma and renal ceramide levels were significantly increased 8 weeks after STZ administration. In the liver and kidney, acid and neutral sphingomyelinase (SMase) activities were not increased, while secretory sphingomyelinase (sSMase) activity was increased in the plasma of diabetic rats after STZ administration. These data indicated that STZ administration induced the increase in plasma ceramide levels via the increase in sSMase activity. It was suggested that increased plasma ceramide levels were involved in the renal damage induced by STZ in diabetic rats accompanied with the enhancement of oxidative stress.