This study compared the status of brain mitochondria isolated from 12-week streptozotocin (STZ)-diabetic rats versus STZ-diabetic animals treated with insulin during a period of 4 weeks. Brain mitochondria isolated from 12-week citrate (vehicle)-treated rats were used as control. For that purpose, several mitochondrial parameters were evaluated: respiratory indexes (respiratory control ratio (RCR) and ADP/O ratio), transmembrane potential (DeltaPsim), repolarization lag phase, repolarization level, ATP, glutathione and coenzyme Q (CoQ) contents, production of H2O2, ATPase activity, and the capacity of mitochondria to accumulate Ca2+. Furthermore, the effect of Abeta1-40 was also analyzed. We observed that STZ-induced diabetes promoted a significant decrease in mitochondrial CoQ9, ATPase activity, and a lower capacity of mitochondria to accumulate Ca2+ when compared with control and insulin-treated diabetic rats. The presence of 4 microM Abeta1-40 induced a significant decrease in RCR in the three groups of rats. However, this peptide induced a significant increase in the repolarization lag phase and a significant decrease in the repolarization level in control and diabetic animals without insulin treatment. Furthermore, this peptide exacerbated significantly the production of H2O2 in STZ-diabetic rats, this effect being avoided by insulin treatment. Our data show that although diabetes induces some alterations in brain mitochondrial activity, those alterations do not interfere significantly with mitochondria functional efficiency. Similarly, insulin does not affect basal mitochondria function. However, in the presence of amyloid beta-peptide, insulin seems to prevent the decline in mitochondrial oxidative phosphorylation efficiency and avoids an increase in oxidative stress, improving or preserving the function of neurons under adverse conditions, such as Alzheimer's disease.