Oxidative stress is an important concept in biology and medicine and refers to a condition where the levels of reactive oxygen species (ROS) significantly overwhelm the capacity of antioxidant defenses, leading to potential damage to the body. ROS levels fluctuate during a 24-hour cycle due to internal circadian rhythms. ROS molecules play a key role in the organism, as they take part in signaling pathways in the cell, but could also trigger the onset of different pathophysiological conditions. For this reason, it is important to have a method that is able to accurately measure ROS concentrations. In this thesis we tested two commonly used dyes: dihydroethidium (DHE) and 2',7'-dichlorodihydrofluorescein diacetate (H2DCF-DA), in different buffers and pH levels. We found that DHE is a more sensitive dye for measuring H2O2 concentration in Drosophila melanogaster (D. melanogaster) samples. H2DCF’s inadequacy was explained by the lack of Fe2+/Fe3+ needed for the reaction to occur, since H2DCF-DA does not directly react with H2O2. DHE is both very stable and able to directly react with H2O2. These experiments also showed how there was a significantly higher H2O2 concentration in the head than in the whole body extracts, but we did not notice a difference between samples collected in the morning and evening. The reason for higher ROS level in the head extracts is due to high levels of oxidizable molecules and fairly low levels of antioxidant defense compared to the rest of the body. Our experiments will contribute to a better understanding of the regulation of ROS concentration, both as a function of the time of day and as a function of different environmental stressors or pathologies.