A totally closed feed back controlled anesthesia- and ventilation circuit has been developed feasible to be applied for artificial ventilation with tidal volumes as low as 5 ml at a rate of up to 60/min and for spontaneous ventilation with on-line measurement of physiological lung parameters (pressure, volume, flow). Oxygen inflow is regulated via actual-set value comparison, oxygen inflow is measured and recorded on-line (= oxygen uptake by the connected subject). On-line oxygen uptake (consumption) measurement furnishes a valuable, so far not available parameter to monitor changes in the oxygen transport chain to the tissue and to register physiological oxygen consumption values and derangement of metabolism. First results show that total body oxygen consumption of man in rest is lower than so far expected in the high weight and body surface area groups and that oxygen consumption is decreasing with length and age. Metabolic derangements such as in developing hyperthermia crisis are noticed in a very early stage when therapy is still possible before severe damage has occurred.