Up to now there is no fully functional and precise technique for accurate assessment for monitoring the degree of unconsciousness during general anaesthesia. Previous studies and results obtained during the project regarding the effect of anaesthetics on single neurons and small neuronal networks indicate that it would be most promising to use a method directly correlating to the integrity of the relevant functional systems. As one goal of the project it was formulated to develop and test a new system for recording and analysis of mid latency auditory evoked potentials (MLAEP), suitable to be used in the clinical surrounding of an operation theatre. Step by step an investigational system was developed, that after obtaining legal approvement was functional in a broad spectrum of cases. In a multicenter study the including 4 clinical centres and 426 patients the reliability of the system was proved. Even under the hard conditions of the OR the apparatus was able to record the signals with sufficient reliability and precision. A clear correlation of MLAEP parameters with clinically defined states of consciousness could be identified. Differences in the effect of the used combinations of general anaesthetics and opioids were identified. An algorithm for automatic analysis of the signals was developed, to allow users with limited knowledge of interpretation of MLAEP signals to use them for monitoring anaesthesia.