Postnatal nephrogenesis and growth of the rat kidney is greatest during the first 10 days of life. The renin-angiotensin system has important growth-promoting effects. To explore the relationship between postnatal renal development and the renin-angiotensin system, we examined the steady-state levels of messenger ribonucleic acid (mRNA) for renin, angiotensinogen, histone H2b (a marker of DNA synthesis) and the structural protein actin. The pattern of expression for renin and histone H2b were similar, with maximal levels of both mRNAs occurring during the first 10 days of life, the time of most marked postnatal nephrogenesis and kidney growth. A different pattern was seen for angiotensinogen mRNA, with low to undetectable levels during the first 15 days of life, and for actin mRNA, with fairly stable expression at the time points examined, providing evidence that the increase in renin and histone H2b mRNA was not due to a nonspecific increase in mRNA in the newborn kidney. In conclusion, the increase in renal histone H2b mRNA during the early postnatal period is consistent with this being a time of increased DNA synthesis. The similar time course for renin mRNA, histone H2b mRNA and previously defined changes in postnatal nephrogenesis suggests a role for the renin-angiotensin system in postnatal nephron formation.