"Several investigators have shown that diminished sinus arrhythmia can be seen as an indication of increased mental load. The present experiment deals with the influence of different levels of mental load, operationalized as the number of binary choices per minute, on the regularity of the heart rate. Also . . . investigated -were= the influence of 4 different rest conditions on the regularity of the heart rate. The results show that sinus arrhythmia scores differentiate significantly between several levels of mental load, but heart frequency appeared to be an even better indicator. No significant differences were found between the 4 rest conditions. Stability over 3 subsequent measurement sessions was satisfactory, and reliability within the measurement periods was very high."