Streptozotocin (STZ) is used extensively to induce pancreatic -cell death and ultimately diabetes mellitus in animal models. However, the direct effects of STZ on muscle are largely unknown. To delineate the effects of STZ from the effects of hypoinsulinemia/hyperglycemia, we injected young rats with (i) saline (control), (ii) STZ (120mg/kg) or (iii) STZ & Insulin (STZ-INS; to maintain euglycemia). STZ rats demonstrated significantly elevated blood glucose throughout the 48 hour protocol, while control and STZ-INS rats were euglycemic. Body mass increased in control (13 ± 4g), decreased by 19 ± 2g in STZ and remained unchanged in STZ-INS rats (-0.3 ± 2g). Cross-sectional area of gastrocnemius muscle fibres were smaller in STZ versus control (1480 ± 149 vs. 1870 ± 40µm², respectively; p0.05) and insulin treatment did not rescue this defect (STZ-INS: 1476 ± 143µm²). Western blot analysis revealed a detectable increase in ubiquitinated proteins in the STZ skeletal muscles compared to control and STZ-INS. To further define the effects of STZ on skeletal muscle, independent of hyperglycemia, myoblasts were exposed to varying doses of STZ (0.25-3.0mg/ml) in vitro. Both acute and chronic exposures of STZ significantly impaired proliferative capacity in a dose-dependent manner. Within STZ-treated myoblasts, increased reactive oxygen species (ROS) was associated with significant G₂/M phase cell-cycle arrest. Taken together, our findings show that the effects of STZ are not -cell specific and reveal that STZ should not be used for studies examining diabetic myopathy.