The effect of different dosages of streptozotocin (STZ) on selected rat tissue enzyme activities and glycogen concentration were investigated. The rats were administered STZ intravenously at 60 (STZ-60), 80 (STZ-80), 100 (STZ-100), and 150 (STZ-150) mg/kg body weight. They were used 3 weeks postinjection. Mortality prior to kill occurred only in the STZ-100 and STZ-150 rats. All diabetic rats showed reduced growth rate, hyperglycemia, hypoinsulinemia, and hyperlipemia. Phoshofructokinase (PFK) and succinate dehydrogenase (SDH) activities were significantly reduced in the red gastrocnemius muscle of all diabetic rats, and in the white gastrocnemius and soleus of STZ-100 and STZ-150 groups. PFK activity in the heart remained unaltered, but SDH activity was below normal. Liver SDH activity was not affected by insulin deficiency. Glycogen content was markedly increased in the heart and decreased in the liver of all diabetic rats. Glycogen content in the skeletal muscle was similar to the controls, except for the lower values in the soleus of STZ-100 and STZ-150 rats. When STZ-80 and STZ-150 rats were given insulin therapy, the STZ-80 rats showed a greater response to the treatment. Despite similar levels of plasma immunoreactive insulin among all groups of diabetic rats, the STZ-100 and STZ-150 rats had higher mortality, greater loss in body weight, and alterations in enzyme activities and glycogen content in the tissues studied.