To investigate the potential role of angiotensin II (Ang II) type 1 receptors (AT(1)Rs) as well as endogenously produced nitric oxide (NO) in regulating cardiovascular homeostasis during ontogeny, experiments were carried out in conscious lambs aged approximately 1 week (N = 9) and 6 weeks (N = 11). The arterial baroreflex control of heart rate (HR) was assessed before and after intravenous (IV) infusion of the selective AT(1)R antagonist, ZD 7155, before and after IV administration of the L-arginine analogue, N(G)-nitro-L-arginine methyl ester (L-NAME). In both groups, after ZD 7155 alone, mean arterial pressure decreased then increased after L-NAME. At 1 but not 6 weeks, HR decreased after ZD 7155 as well as after L-NAME. At 1 but not 6 weeks, there was a decrease in the HR range after ZD 7155 and after ZD 7155 + L-NAME, as compared to control. There was also a decrease in minimum HR after ZD 7155 + L-NAME at 1 week. These data provide new evidence that, together, Ang II and NO regulate cardiovascular homeostasis as well as the arterial baroreflex of HR early in life which may help to explain the activation of these two systems early in life.