Theories of emotion regulation and social engagement indicate that resting and reactive respiratory sinus arrhythmia (RSA), which reflect vagal activity, in early childhood can inform the development of social and emotional behaviors later in life. Low RSA at baseline and during a stressful task have been associated with symptoms of ED and disorders characterized by social impairments. The current study examined the mediating role of ED at 24-months-old (mo) on the prospective association between early infant resting and reactive RSA at 5 mo on social functioning outcomes at 48 mo, and the mediating role of social functioning at 24 mo on infant RSA at 5 mo and ED at 48 mo in 237 healthy children. It was hypothesized that ED would mediate the relationship between infant RSA and later childhood social function. Results largely indicated no support of the hypotheses and that there is no mediating effect of childhood ED on early RSA on later childhood social behavior; however, there was a significant relationship between infant resting and reactive RSA and later ED. Limitations and future directions for improving the methodology are addressed.