Reactive oxygen species (ROS) generated by the NADPH oxidase have been proposed to play an important role in the carotid body (CB) oxygen sensing process (Cross et al. 1990). Up to now it remains unclear whether hypoxia causes an increase or decrease of CB ROS levels. We transfected CBs with the ROS sensitive HSP-FRET construct and subsequently measured the intracellular redox state by means of Förster resonance energy transfer (FRET) microscopy. In a previous study we found both increasing and decreasing ROS levels under hypoxic conditions. The transition from decreasing to increasing ROS levels coincided with the change of the caging system from ambient environment caging (AEC) to individually ventilated caging (IVC) (Bernardini A, Brockmeier U, Metzen E, Berchner-Pfannschmidt U, Harde E, Acker-Palmer A, Papkovsky D, Acker H, Fandrey J, Type I cell ROS kinetics under hypoxia in the intact mouse carotid body ex vivo: a FRET based study. Am J Physiol Cell Physiol. doi: 10.1152/ajpcell.00370.2013 , 2014). In this work we analyze hypoxia induced ROS reaction of animals from an IVC system that had been exposed to AEC conditions for 5 days. The results further support the hypothesis of an important impact of the caging system on CB ROS reaction.