Autonomic responses were measured while 45 adult women performed a standard experimental stress task in the laboratory with only the experimenter present and 2 weeks later at home in the presence of a female friend, pet dog, or neither. Results demonstrated that autonomic reactivity was moderated by the presence of a companion, the nature of whom was critical to the size and direction of the effect. Ss in the friend condition exhibited higher physiological reactivity and poorer performance than subjects in the control and pet conditions. Ss in the pet condition showed less physiological reactivity during stressful tasks than Ss in the other conditions. The results are interpreted in terms of the degree to which friends and pets are perceived as evaluative during stressful task performance. Physiological reactivity was consistent across the laboratory and field settings. Because individuals who experience pronounced, frequent, or enduring autonomically mediated cardiovascular responses to stress may be at risk for the development of cardiovascular disease (Clarkson, Manuck, & Kaplan, 1986; Manuck & Krantz, 1986), psychological variables that mediate or moderate autonomic reactivity to stress are important to consider. Nearly all research on psychological moderators of autonomic reactivity has focused on personality variables, such as coronary-prone behavior type, hostility, anger, aggression, anxiety, and denial (Houston, 1986). Relatively little attention has been paid to social psychological constructs such as attitudes and relationships. This study focused on the presence of others as a potential moderating variable in stressful situations. Specifically, we were interested in the degree to which potentially evaluative and nonevaluative others could act as buffers of autonomic reactivity during a stressful situation. In addition, we were interested in a comparison of laboratory and field sites. Social facilitation theory (Zajonc, 1965,1980) posits that the presence of others increases arousal, which in turn increases or decreases performance as a function of the degree to which task requirements are based on well-learned dominant responses. Zajonc's (1965) social facilitation formulation posited that the mere presence of others increases an individual's arousal and affects performance. Recently, Cacioppo, Rourke, Marshall-Goodell, Tassinary, and Baron (1990) clarified the impact of the presence of others on arousal by demonstrating that the presence of others does not increase basal levels of physiologi-This research was supported by Grant BRSG S07 RR 07066 awarded by the Biomedical Research Support Program, National Institutes of Health, to Karen M. Allen. We express our appreciation to several anonymous reviewers for their comments on a previous draft of this article.