The aim of this study was to evaluate whether psychological stress causes increased muscle activity, especially static muscle activity, in the trapezius muscles of the neck and shoulder. A pilot and a main study were carried out with 10 males and 20 females, respectively. The subjects performed a VDU data entry task under psychologically stressful and non-stressful conditions. Stress reactions were measured by different methods : heart rate and heart rate variability, hormonal excretion, and subjective ratings of mood and body symptoms. The stress condition caused an increase in heart rate. The low-frequency variability increased and the high-frequency variability decreased. Ratings of motivation and relaxation decreased, and subjects felt more activated. Pain and discomfort from the stomach increased. Adrenaline and noradrenaline in urine were unaffected. The majority of the subjects showed moderately increased static and median EMG levels during the stress condition. The interindividual variation in muscular reactions was large. Statistically significant increases were obtained for the myoelectric activity of the left (resting) trapezius muscle when pooling the two groups. It appears that the increase in muscle activity due to this type of mental stress is small, and factors other than ' attention-related ' load may be more important. The results indicate that some individuals may be more prone to general muscle tension, making them more likely to develop symptoms and musculoskeletal pain.