This study examines the moderating role of 12-month-old infants’ baseline respiratory sinus arrhythmia (RSA; parasympathetic activity linked to children's flexible regulation) in the relation between temperamental negative affect (NA; sadness, anger, fear) in parents and infants. Among the 51 infants included in the initial analysis, we found a non-significant, trending interaction between baseline RSA and parent NA and no significant main effects of parent NA on child NA. Additional analyses were performed, showing significant results.