The relation between the use of defense mechanisms and autonomic nervous system reactivity, under conditions of laboratory stress, was studied in 78 men and women. Both diastolic blood pressure (DBP) and skin conductance level (SCL) were monitored during exposure to ten stress tasks; concurrently, the use of three defense mechanisms was assessed by coding Thematic Apperception Test stories. Autonomic reactivity was found to be related to defenses; the nature of that relation differed across the defenses. DBP, typically found to be associated with cognitive work, was higher in those individuals who used more Identification, a defense that requires greater cognitive activity. The use of Projection, on the other hand, was associated with lower DBP. In addition, the use of Identification showed a tendency to be associated with lower SCL.