Semicarbazide-sensitive amine oxidase (SSAO) activity was investigated in serum and tissues of streptozotocin diabetic rats. Diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ, 60 mg/kg) and rats were killed at 1, 3, 7, 9, 14, 28 and 56 days after treatment. STZ increased serum glucose and serum SSAO activity at all time points with a maximal increase (2 to 3 fold) at day 7. Kidney SSAO activity showed significant increases on days 9 and 14 (2.5 and 4 fold, respectively). SSAO activity of aorta, lung and pancreas was not changed significantly. Kinetic analysis showed that the elevation in both serum and kidney SSAO activity was due to an increased Vmax with no change in Km. SSAO activity appears to be selectively increased in the serum and kidney of STZ diabetic rats which may be indicative of a relationship between serum SSAO and early renovascular damage in this animal model of diabetes mellitus.