We studied changes in the patterns of heart rate variability (HRV) that coincide with the development of diving skills in harbor seal pups, Phoca vitulina. Heart rate measurements were collected remotely. Spectral analysis of HRV revealed power within a mid-frequency band (0.1–0.3 Hz) which was prominent, especially in pups less than 10 d of age. In these younger pups, the heart rate switched cyclically between a low and a high diving heart rate every 3–10 sec. Older pups exhibited a highly controlled diving bradycardia with a lower median and a lower variance when compared to younger individuals. These results provide new insight into the maturation of the bradycardia component of the dive response in harbor seal pups.