The EEG activity was followed during simulated diving to 20 m depth. Altogether 49 EEG records of 5 min length were taken in 5 divers breathing compressed air in a close system. No significant changes of the EEG could be found during either compression or decompression if only conventional indicators, such as the amplitude of theta activity, were tested. However, a complex "normality ratio," based on multivariate analysis of a normal material, was capable of showing significant impairment of the brain activity during the decompression. The indicator has been designed to reflect the abnormality level rather than the short-lasting EEG changes, the latter being mostly dependent on the alertness fluctuations. The EEG changes associated with alertness fluctuations were assessed separately by means of a specific "alertness indicator" and a prominent alertness increase was found during both compression and decompression phases of the diving experiment.