With a view to developing a method of evaluating mental work stress, a paced calculating task (addition and substraction of two two-digit figures) with eight grades of workload (20%-160% of work capacity) was assigned to 21 male and 20 female students. Using a VDT, subjects performed the task and psychological and physiological measures of mental activity (intellectual activity, feeling, arousal level) were determined. Occipital midline (Oz) beta 2 (20 Hz-30 Hz) amplitudes regressed linearly against workloads, with a marked rise observed at overload levels (above 100% workload), though the number of correct responses did not increase at such levels. Error rate, subjective task difficulties, and feelings (unpleasant, tense, excited) were proportional to workload and closely correlated to each other. Blood pressures and heart rates increased slightly in proportion to workload, with a correlation between them. Feelings were presumed to be not particularly intense even at overload levels. Regression coefficients of error rate, excitement level, and Oz beta 2 amplitude were larger in females than males. Based on the results, it was inferred that females might sustain somewhat more severe work stress than males. Critical flicker frequencies decreased over time. The criteria for evaluating heavy and excessive mental work stresses were deduced from the Oz beta 2 amplitudes, which best reflected intellectual activity, and from the critical flicker frequency, which is related to arousal level.