This study examines the possibility that social support operates as a moderator of cardiovascular reactivity, which may be a factor in the development of heart disease and hypertension. An experiment was performed in which each of 40 subjects was the object of verbal attack in a discussion of a controversial issue. In each session, one subject and three confederates participated. Two of the confederates argued with the subject; in half the groups, a third confederate defended the subject's position (social support condition); in the other half, the third confederate sat quietly (no support condition). The subject's blood pressure and heart rate were continuously monitored. Subjects in the social support condition showed significantly smaller increases in cardiovascular measures than subjects in the no support condition. The results are discussed in terms of small group dynamics and Social Comparison Theory.