We examined cardiovascular responses indicating challenge and threat during social comparisons. Experiment 1 manipulated comparison direction (i.e. upward/downward) within a cooperative social interaction, during which we measured cardiovascular responses, evaluations of demands and resources, and self‐reports. Participants interacting with upward comparison partners evaluated the task as more ‘threatening’ (demands relative to resources) than participants cooperating with downward comparison partners. Moreover, participants cooperating with upward comparison partners exhibited cardiovascular reactivity consistent with threat (i.e. increased ventricle contractility, no changes in cardiac output, and vasoconstriction). In contrast, participants interacting with downward comparison partners exhibited challenge responses (i.e. increased contractility, increased cardiac output, and vasodilation). This basic finding was extended in Experiment 2 with the examination of a classic moderator of social comparison, attitudinal similarity of the comparison partner. Participants paired with attitudinally dissimilar partners exhibited exacerbated reactions relative to participants paired with attitudinally similar partners. That is, relative to similar partners, dissimilar partners engendered greater threat responses during upward comparisons and a tendency toward greater challenge responses during downward comparisons. These results are discussed within an assimilative/contrast model of social comparisons. Copyright © 2001 John Wiley & Sons, Ltd.