Regular exercise is said to have positive effects on mood, especially if the exercise intensity is low to moderate. However, the acute effects resulting from participation in a strenuous competition, such as a marathon race, have been studied less. The present investigation used the Profile of Mood States (POMS) test to measure mood, before and after the 1989 Stockholm Marathon. A total of 106 male runners (mean age 40.0 years), with finishing times between 3h and 3h 45 min participated as subjects. Results showed great changes between pre- and post-marathon scores, most of them significant at the p less than 0.001 level. Furthermore, differences between a faster and a slower group of runners were demonstrated with regard to mood states, even though plasma glucose levels were comparable. It is concluded that participation in a marathon race greatly effects mood, mainly in a more negative way than low to moderately intense exercise does.