Seasonal variation in mood and behavior was examined in 87 American men and women who spent the 1991 austral winter at three different research stations in Antarctica. The South Pole station (90 degrees S) crew reported a significant decline in tension/anxiety, depression, anger, confusion, and fatigue from March to August, followed by a significant increase in tension/anxiety and fatigue and a significant decline in vigor from August to October. The McMurdo station (78 degrees 51' S) crew also reported a significant decline in tension/anxiety from March to July and a significant increase in tension/anxiety from July to August. In contrast, the Palmer station (64 degrees 46' S) crew experienced no significant changes in any mood subscale from May to October. The nonlinear pattern of change in mood suggests that adaptation to prolonged isolation and confinement in an extreme environment occurs in two or three stages.