The purpose of this study was to develop an instrument to measure a person's exercise of self-care agency. Reliability and validity were established. Content validity was established through a rating of each item of the questionnaire by five nursing experts practicing under the self-care concept. The Adjective Check List and Rotter's Internal-External Locus of Control of Reinforcement Scale were used in establishing construct validity. Subjects consisted of 84 associate degree nursing students and 153 psychology students. Results showed (a) a positive correlation of self-confidence, achievement, and intraception with exercise of self-care agency and (b) a negative correlation between abasement and exercise of self-care agency. The test-retest reliability was .77 for the nursing students; split-half reliabilities were .80 and .81, respectively, for the first and second testings of the nursing students, and .77 for the psychology students.