Twelve male swimmers were studied psychologically before, during, and after 10 d of increased training. Daily training distance was increased from 4,000 to 9,000 m.d-1, and intensity was maintained at 94% of VO2max. Three of the swimmers were unable to tolerate the increased training load, and they did the same distance at slower speeds. Swimmers completed the Profile of Mood States, a muscle soreness scale, and a 24-h history each morning prior to the first of two daily training sessions. Changes across days were evaluated statistically with a one-way repeated measures ANOVA. Significant (P less than 0.005) increases occurred in the ratings of exercise intensity, muscle soreness, depression, anger, fatigue, and global mood disturbance, along with a reduction in general sense of well-being. Swimmers were classified as "responders" or "non-responders" on the basis of distress patterns using separate physiological and psychological criteria, and these classifications were performed in a double-blind setting. Close agreement (89%) was achieved between the psychometric and physiological judgments, and the physiological results appear in related papers. It is concluded that significant psychometric changes occur with an intense 10-d training regimen, and these alterations resemble those observed in swimmers exposed to increased training across several months. These findings underscore the potential utility of monitoring mood states in the prevention of staleness.