To determine whether angiotensinogen (Ao) and renin are synthesized by the immature kidney and to assess the changes in intrarenal renin distribution that occur with maturation, the kidneys from 24 newborn and 12 adult Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) were processed for renin immunocytochemistry using a highly specific anti-rat renin antibody. Kidney renin and Ao relative mRNA levels (mRNA/total RNA) were detected by Northern and dot blot techniques, using full-length rat renin and Ao cDNAs. Renal renin concentration (RRC) was measured by radioimmunoassay of angiotensin I (ANG I) and expressed as ng ANG I.h-1.mg protein-1 in the incubation media. RRC was higher in newborn than in adult SHR (979 +/- 164 vs. 206 +/- 47) and WKY (573 +/- 69 vs. 297 +/- 74) (P less than 0.05). In the newborn kidneys of both rat strains, renin was distributed throughout the entire length of the afferent arterioles and interlobular arteries, whereas in the adult kidneys renin was confined to the classical juxtaglomerular position. With maturation, there was a decrease in the proportion of immunoreactive juxtaglomerular apparatuses and arterial segments that contained renin. Kidney renin mRNA levels were 7.9-fold higher in the newborn than in the adult animal. Ao mRNA was detected in the newborn and adult kidneys of both rat strains. This study demonstrates conclusively that both renin and Ao genes are expressed in the newborn kidney, providing evidence for a local renin-angiotensin system that is subjected to developmental changes.