Three studies investigated the reliability and construct validity of the Exercise Thoughts Questionnaire (ETQ), an instrument developed to assess the frequency with which individuals have thoughts involving reasons or excuses for not exercising at the present time. Such cognitions are hypothesized to interfere with exercise behavior. Study 1 involved 164 college women; Study 2, 209 undergraduates; and Study 3, 196 undergraduates. Analyses revealed that the ETQ has good internal consistency and test-retest reliability. ETQ scores related in theoretically meaningful ways to exercise intentions, previous exercise experience, the number of days participants considered exercising but did not actually exercise, and both concurrent and prospective self-reports of exercise behavior. Exploratory analyses revealed that women reported a higher frequency of thoughts involving reasons or excuses for not exercising than men and that students who participated in collegiate, intramural, or club sports having required practices reported a lower frequency of such thoughts.