Summary: The present investigation was undertaken to find the differences, if any, in the pattern of nephrotoxic acute renal failure (HgCl2,4.7 mg/kg body weight SC), in the developing rat and its relationship to the renin angiotensin system. No differences in renal cortical renin content were found between 2, 4, and 8 week olds, but plasma renin concentration was highest at 2 weeks and declined with age. Plasma renin was significantly increased in all groups 6 hr after HgCl2 injection, and the percentage of increase was highest in the 4 week olds. Despite these differences in initial plasma renin and in changes in plasma renin after HgCl2, the pattern of acute renal failure (as assessed by changes in blood urea nitrogen) was similar in the three groups for the first three days. Subsequently, the 4 and 8 week olds exhibited recovery (blood urea nitrogen began to decline), wheras blood urea nitrogen continued to increase to the fifth day in the 2 week olds. The mortality was highest in this group. No simple correlation was observed between basal renal renin, plasma renin, the increase in plasma renin following HgCl2 injection, and the pattern or severity of acute renal failure. Speculation: The delayed recovery of renal function in younger rats may be due to a limitation in their ability to eliminate nephrotoxin imposed by immaturity of both glomerular and tubular function.