This study examined 47 office workers during a day of regular work in front of a video display unit (VDU) and a day of leisure in the same environment. The study investigated possible effects of VDU work on circulating melatonin and adrenocorticotropic hormone (ACTH) levels. Circulating melatonin levels decreased significantly during VDU work, whereas ACTH levels increased significantly. In contrast, melatonin and ACTH levels did not change significantly during a day of leisure. Mental strain during work was significantly and positively associated with circulating levels of ACTH but not melatonin. The results indicate that the VDU environment is associated with measurable changes in melatonin and ACTH levels. Mental strain might explain changes in ACTH levels, but specific factors that might contribute to changes in melatonin levels are unknown. These findings might be relevant to the recent debate about the possible influence that electromagnetic and VDU environments might have on electromagnetic sensitivity.