It was hypothesized that nitric oxide (NO) and prostaglandins (PGs) play a synergistic role in modulating haemodynamic responses to angiotensin II (ANG II) in an age-dependent manner. To this end, experiments were carried out in conscious, chronically instrumented lambs aged ∼1 week (N = 9) and ∼6 weeks (N = 10) to evaluate the haemodynamic responses to ANG II, before and after treatment with the L: -arginine analogue, N-nitro-L: -arginine methyl ester (L: -NAME), as well as the cyclooxygenase inhibitor, indomethacin (INDO). Pressor and renal blood flow responses to ANG II were measured before (control) and after administration of L: -NAME (20 mg kg(-1)), following pretreatment with either vehicle (VEH) (experiment 1) or INDO (1 mg kg(-1), experiment 2). The two experiments were carried out at minimum intervals of 48 h. In both age groups, the pressor and renal vasoconstrictor responses to ANG II were augmented by pretreatment with INDO, the effects being similar at 1 and 6 weeks. The haemodynamic responses to ANG II were, however, not altered after L: -NAME following pretreatment with either VEH or INDO. These data provide new evidence that soon after birth, endogenously produced PGs, but not endogenously produced NO, balance the vasoconstrictor actions of ANG II. There is, however, no apparent interaction between PGs and NO in modulating the responses to ANG II postnatally.