Background/Aims: Angiotensin II (Ang II) is an important growth factor in the fetal kidney. Molecular cloning and pharmacological studies have defined two major classes of Ang II receptors designated AT1 and AT2. Two AT1 isoforms, AT1A and AT1B, exist in rodents. AT1 promotes growth and proliferation and mediates many of the known physiological actions of Ang II. AT2 appears to antagonize AT1. Our goal was to measure their relative contributions to Ang II signaling in the developing kidney. Methods: We used real-time RT-PCR to quantify AT1A, AT1B, AT2 and the housekeeping gene EF1α mRNA in kidneys from embryonic (E) day 14–20 and postnatal (P) day 1–14 rats. Results: AT2 mRNA declined from 1.4 × 10<sup>4</sup> copies/10<sup>6</sup> copies EF1α on E14 to 4.2 × 10<sup>3</sup> copies/10<sup>6</sup> copies EF1α on P14. In contrast, total AT1 mRNA increased gradually from 2.0 × 10<sup>3</sup> copies/10<sup>6</sup> copies EF1α on E14 to 2.0 × 10<sup>4</sup> copies/10<sup>6</sup> copies EF1α on P14, with AT1A accounting for about 90% of total AT1 mRNA throughout nephrogenesis. Moreover, the ratio of AT2/(AT1A + AT1B) decreased in a log-linear fashion during maturation, from 6.7 on E14 to 0.2 on P14. Conclusion: The ratio of AT2 to AT1 gene expression modulates Ang II action in the developing kidney.