Elevation of intraocular pressure (IOP) in primary open angle glaucoma is hypothesized to be influenced by stress-induced activation. However, empirical support for this assumption so far is only small, and the amount of stress effects has not yet been determined as compared to other known factors that influence IOP-levels. This study investigated the impact of a mental stressor test (MST) on the IOP of open angle glaucoma patients as compared to IOP reactions provoked by the Water Drinking Test (WDT), which is a valid diagnostic procedure to detect maximum IOP levels. Assessments of cardiovascular reactions and self-ratings of psychological strain showed the MST was effective in altering general psychophysiological activation. immediately after exposition to the mental stressor, an increase of IOP of about 1.5 mmHg on average was determined. After the WDT, averaged elevations of IOP amounted to 4 mmHg. After a 10 minutes relaxation phase during the MST, IOP scores had returned to baseline-levels. During the WDT, IOP also decreased within a 40 minutes period but still exceeded baseline-levels. The results provide evidence that psychosocial stress factors may contribute to increased IOP in open angle glaucoma, although the WDT led to more pronounced elevations of IOP and a prolonged recovery as compared to the MST.