Circadian rhythm is present in almost all living organisms due to the rotation of the Earth and regular light and dark changes. Its role is evident in 24 hour modulation of biological processes such as the sleep/wake cycle. Disrupted circadian rhythm correlates with oxidative stress, which is caused by increased production of Reactive Oxygen Species (ROS). In this thesis, we explored the connection between the circadian rhythm and oxidative state in D. melanogaster. Based on similarity in the responsiveness to volatilized cocaine between flies with mutation in timeless gene and wild type flies, we were interested to see whether these similarities will be paralleled in their redox regulation. An indicator of redox state was the concentration of the oxidative metabolism byproduct H2O2 in head homogenates. For detection of H2O2 we used the dihydroethidium (DHE) staining. Furthermore, we wanted to determine if H2O2 production is regulated by circadian rhythm. Measurements were conducted in flies raised in conditions of 12 hours of light-12 hours of dark (LD) and in conditions of constant dark (DD). We determined that H2O2 production was regulated by circadian rhythm for wt flies. However, H2O2 production in DD conditions was increased overall. In tim mutants there was an indication of circadian regulation of H2O2 production in LD conditions but the rhythm was abolished in DD conditions. wt and tim flies differed significantly in the 24 hour modulation of H2O2 in DD, but less so in LD. Further studies are needed to elucidate the role of tim in the regulation of H2O2 amount and the connection with cocaine responsiveness. Taken together, our data suggest that redox status is regulated in a circadian fashion and that a functional tim gene is essential for maintaining circadian rhythmicity.