Respiratory sinus arrhythmia (RSA) figures prominently in emotional responding, but its exact role remains unclear. The present study tests two hypotheses: (1) Between-person differences in resting RSA are related to emotional reactivity, and (2) within-person changes in RSA are related to regulatory efforts. Pairs of women watched an upsetting film and discussed it. One woman in each of the experimental dyads was asked to either suppress or to reappraise during the conversation. Their partners and both members of the control dyads conversed naturally. Between-person differences in resting RSA were assessed with paced breathing, and within-person changes in RSA were calculated from baseline to the conversation accounting for respiration. Women with higher resting RSA experienced and expressed more negative emotion, and women who attempted to regulate their emotions either by suppressing or reappraising showed larger increases in RSA than controls.