The present review focuses on our current understanding of the arterial baroreflex control of heart rate in the adult as well as during development and includes recent findings regarding the role of nitric oxide (NO) and angiotensin II (ANG II) in modulating this reflex.  Our experiments carried out in conscious, chronically-instrumented lambs have provided evidence that the arterial baroreflex is operating at its upper range soon after birth, whereas later in life, when resting heart rate is lower, the arterial baroreflex operates mid-range. We have also shown that endogenously produced ANG II and NO modulate resting heart rate and arterial pressure as well as the arterial baroreflex control of the heart in an age-dependent manner, effects of both factors predominating in the newborn period.  Further studies will be important in determining the potential interactions between these two factors, as well as the underlying mechanisms.  Advances in our understanding of the reflex control of the circulation during postnatal maturation in conscious developing animals will provide a scientific basis for understanding the physiology of the developing human infant.