To test the hypothesis that angiotensin (ANG) II modulates the arterial baroreflex control of heart rate (HR) in an age-dependent manner, various parameters governing the arterial baroreflex control of HR were assessed before and after removal of endogenously produced ANG II by administration of the angiotensin-converting enzyme (ACE) inhibitor, captopril, to conscious, chronically instrumented lambs aged ~1 week (8 +/- 1 days; n = 8) or ~6 weeks (46 +/- 5 days; n = 8). After administration of captopril, systolic, diastolic and mean arterial pressures decreased significantly from control levels and HR increased; however, the effects were greater in 1- than in 6-week-old lambs. In 1-week-old lambs, after administration of captopril, there was also a significant increase in the slope coefficient, a decrease in minimum HR and a decrease in the point of maximum gain. In 6-week-old lambs, there were no effects of captopril on any of the parameters governing the arterial baroreflex. Therefore, we accept our hypothesis and conclude that the role of ANG II in modulating cardiovascular homeostasis appears to be more predominant in the newborn than later in life. Experimental Physiology (2003) 88.6, 761-768.