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Free radicals are created when exogenous noxae act on the skin. In the defence against radicals, antioxidants form a safety chain which decomposes ROS and prevents injuries. In this study measurements have been taken on the abdominal skin of 6 human beings, the skin of 14 cow’s udders and the skin of 6 pig’s ears to detect β-carotine, catalase enzyme activity and oxygen levels in the skin. This study demonstrates that subsequent to the influence of the same stresses, the formation of radicals is significantly reduced in vitro compared to the situation in vivo. Another important finding is that the in vitro examination is not suited to analyze the kinetics of antioxidants in the human skin or in animal models. Previously published results regarding the influence of exogenous stress factors such as UV-radiation pertain to studies using excised human skin. Insight obtained through these series of tests seem to only be the „tip of the iceberg“, because under in vivo condition, the formation of radicals can be expected to be dramatically higher. The insight gained during the course of the examinations should lead to wide-ranging consequences for the development of protection strategies against UV-radiation. Additionally, the conducted test series show that stress factors which are unsuitable for in vivo testing on human subjects, can indeed be tested on an animal model (e.g. cow’s udder). to To evaluate the sun protection factor in sunscreen for example, humans are up to now exposed to UV radiation to determine erythema effectivity. The focus however should not lie on measuring the kinetics of antioxidants using raman-resonancespectroscopy because no correlation between the measurements in vivo and in vitro could be detected. The formation of free radicals should rather be verified directly with highly sensitive measuring technology, e.g. ESR. In summary, this thesis demonstrates how to examine stress factors in vitro in an animal model, which are unsuitable for testing in vivo. Research on the interactions between antioxidants and free radicals in the human system in vivo and in vitro and in the animal model in vitro conducted through the course of this thesis allows to conclude that relevant understanding of the formation of radicals under in vivo conditions could be gained with more sensible technology like ESR. The results indicate the need for new and stricter standards for the development of protection mechanisms for the neutralisation of free radicals created through various stress factors in the skin in vivo.