Investigations on orthostatic tolerance and physical fitness prior to and after 6-8 h immersion experiments surprisingly revealed that all untrained subjects endured pre- and postimmersion tilt table tests without subjective complaints, whereas after immersion all trained participants collapsed within the first minutes of erect position displaying symptoms of a vasovagal syncope. Assuming that this impaired orthostatic tolerance of trained subjects can partially be referred to influences of an altered blood pressure control system we decided to record the blood pressure characteristics of both groups. Thereby, the transmural pressure of the carotid artery was changed by applying from outside either reduced or exceeding pressures. The controller sensitivity in the untrained group was almost twice as high as in the trained one. This implies a better regulatory response to disturbing interference in the untrained. Under normal conditions the controlling capacity of both groups is sufficient to cope with body position changes. However, during stress situations--in this case, reduction of aldosterone concentration as well as losses of plasma volume due to the 'Gauer-Henry-Reflex'--blood pressure regulation first fails in the trained subjects.