We have previously demonstrated that vagal phase-response curves (PRC), which characterize the effects of critically timed, brief vagal stimuli on sinus node automaticity, exhibit a fundamentally different shape in the canine newborn than in the adult. In this study we analyzed the changes in sinus cycle length in response to critically timed, brief vagal stimuli, delivered to the decentralized cervical right and left vagosympathetic trunks, in two older age groups: 14 1-mo-old puppies (ages 21–36 d), and eight 2-mo-old puppies (ages 56–62 d). Vagal PRC were constructed by plotting the magnitude (percent change) of the vagal chronotropic response as a function of the phase of the cardiac cycle at which the vagus nerve was stimulated. At 1 mo of age adult-type PRC were observed, but in only six of the puppies (43%) and only in response to right vagal stimulation. By 2 mo of age adult-type PRC were observed in seven of eight puppies (88%) in response to right vagal stimulation and in three of eight (38%) in response to left vagal stimulation. Thus, clear developmental changes in the phase dependence of the vagal chronotropic response can be tracked over the first 2 mo of life in the dog.