The effects of physical exercise on psychologic variables during mental stress were evaluated. On each of two different days (for intraindividual control) 20 healthy dental students carried out four (modified) d2-tests (3.5min available for each test). After two such tests there was a rest period of 5 minutes. During this intermission either a standardized physical exercise was performed or -the other day- (cross over, balanced) the volunteers rested in a sitting position while listening to relaxing music. After the rest period another two d2-tests were carried out. Thereafter a questionnaire (“adjective list”, in German) defining 15 subscales (categories of the state of well-being) was completed by all volunteers to quantify parameters of their actual mood. For intraindividual control the results of the subscales obtained on both days were compared by the paired student-t-test. In the test with physical exercise during the break the scores of the subscale being activated were significantly (p<0.05) higher than in the test with music. The other subscales showed no significant difference. In all tests the scores of the d2-tests reflecting the capability to concentrate showed an increase after the break, which was significantly higher, if physical exercise was performed during the break. Physical exercise can alleviate certain symptoms of mental stress. Feeling more active is the predominant subjective effect.