Value of the average level of asymmetry of the EEG waves' fronts (ALA) was shown to decrease in posterior leads and increase in anterior leads during the transition from the rest to mental activity. An inter-hemisphere asymmetry of the ALA was found, the asymmetry having a similar direction for all the pairs of symmetrical leads. Two independent factors are supposed to affect the ALA: the level of the cortex activation and the specifics of information processing by the given hemisphere.