Background: Exposure to anger in the family is a risk factor for disruptive behavior disorders characterized by ineffective vagal regulation. Effects of anger on developing vagal regulation may be due to direct exposure or to effects on parents' regulation of emotion as parents support infants' regulation. Little is known about the impact of anger during infancy when important regulatory systems emerge. Methods: Six-month-old infants (N = 48) and their mothers were exposed to anger, excitement, or neutral affect then observed in the Still-Face Paradigm (SFP). Vagal tone, indexed by respiratory sinus arrhythmia (RSA), was measured. Results: Infants exposed to anger subsequently showed greater RSA withdrawal to mothers' still-face than infants exposed to other emotions. Mothers exposed to anger showed greater RSA withdrawal than other mothers during emotion exposure and across all episodes of the SFP. Conclusions: Exposure to anger may sensitize infants to stress and lead to increased need for physiological regulation. Exposure to anger makes increased demands on mothers' self-regulation, which could detract from their abilities to support infants' regulation.