In vivo exposure therapy for phobias is uniquely suited for controlled studies of endocrine and physiologic responses during psychologic stress. In this study, exposure therapy induced significant increases in subjective anxiety, pulse, blood pressure, plasma norepinephrine, epinephrine, insulin, cortisol, and growth hormone, but did not change plasma glucagon or pancreatic polypeptide. Although the subjective and behavioral manifestations of anxiety were consistent and intense, the magnitude, consistency, timing, and concordance of endocrine and cardiovascular responses showed considerable variation.