End tidal partial pressure of carbon-dioxide (124389) (PCO2), defined as the percentage of peak concentration of carbon-dioxide in a single breath of exhaled air at the end of the expiratory phase, was explored as an indicator of psychophysiologic stress during data entry work. PCO2, respiration rate, heart rates, and self ratings of tension or relaxation were measured in 11 individuals during relaxation exercises and data entry work. Only end tidal PCO2 was able to discriminate between periods of relaxation and data entry work. This result indicated that the reductions observed in end tidal CO2 denoted a hyperventilatory stress effect as opposed to a metabolic effect due to the physical demands of repetitive keying. End tidal PCO2 increased significantly from the baseline relaxation period to the progressive relaxation period, and decreased significantly from the progressive relaxation period to the total work period. End tidal PCO2 for the total work period was significantly lower than that for the baseline relaxation period. Although the average change in end tidal PCO2 was only 1 to 2mmHg, it was consistent with other studies of the effects of stress on respiration. The authors conclude that end tidal PCO2 shows promise as an index of psychophysiological activity in computer tasks. Also, because no differences were detected for end tidal CO2 concentrations between sessions over the course of the day, this parameter may also distinguish psychological effects from circadian rhythm effects.