A modification of mental arithmetic (MMA) combined with a key press technique was used to raise the patient's level of vigilance during routine investigation of vestibular function. The effect of MMA is studied by comparing succeeding pendular tests under different conditions. Group I: 81 patients had the first pendular test without and the second with MMA. Control group II: 31 patients had the tests in opposite order. The nystagmograms were analysed by computer. The application of the MMA-method resulted in a significantly increased average reaction intensity with differences up to 20% and more, according to the considered nystagmus parameter. This effect does not depend on the chronological order of the tests with or without MMA. On the other hand, in both groups a higher level of vigilance is correlated with less nystagmic beats (5-8%). Therefore, the number of beats does not show the characteristics of typical intensity parameters. The comparison of the absolute values of the analysed parameters shows good reproducibility of the average results under identical circumstances. In the last 4 years the MMA-method turned out to be easily applicable. For all these reasons, this method was included into the routine examination of vestibular system function to raise and keep constant the level of vigilance.