Present results show that ether anesthesia raises the mean plasma insulin levels and the variability of the individual data around the mean in rats under different experimental conditions. Previous results [6, 7] obtained in ether-anesthetized rats, showing that the plasma insulin levels of hypothyroid animals depends on the goitrogens used, and that there is a positive correlation between the individual thyroid weight and the plasma insulin levels, are confirmed in samples obtained without anesthesia.