Previous research suggests that cardiac vagal regulation (indexed by respiratory sinus arrhythmia, or RSA) provides a physiological substrate for affect regulation, which presumably underlies adaptive interpersonal functioning.The authors tested these associations in the context of daily interactions between 68 cohabiting couples. Participants underwent a laboratory assessment of RSA during rest and also during a series of psychological stressors. Subsequently, they kept daily measures of affect and interaction quality for 21 days. Individual differences in baseline and stress levels of RSA moderated within-person associations between daily affect and the quality of couple interactions. The pattern of results differed for women versus men. Men with lower vagal tone or higher vagal reactivity had stronger associations between daily negative affect and daily negative interactions, and men with higher vagal tone had more positive daily interactions overall. Women with higher vagal tone had stronger associations between daily positive affect and daily positive interactions.