Using a model of unilateral nephrectomy and contralateral heminephrectomy in weanling rats, we studied the effect of severe (6%) and moderate (14%) protein restriction on growth and renal function up to 24 wk of age. One-half the normal (N) and chronic renal insufficiency (CRI) rats underwent a cross-over protocol 6% — 14% and 14% —» 6% at lOwk of age to analyze the contribution of early growth on ultimate wt and length. N or CRI fed a 6% protein diet were the smallest. Animals switched from 6% — 14% protein diet did not catch up to their counterparts started on a 14% protein diet (N or CRI). However, N or CRI animals started on a 14% protein diet and switched to a 6% protein diet grew along a curve similar to animals fed a 14% protein diet. Creatinine clearances for CRI rats were not different among the groups. However, protein excretion was highest in those animals completing the study on a 14% protein diet. Growth failure caused by severe protein restriction occurred primarily in the early accelerated growth phase (310 wk). We conclude that moderate protein restriction during the early rapid growth phase maintains growth, whereas severe protein restriction is detrimental to growth. Switching to a more restrictive protein intake after the rapid growth phase did appear to offer more renal protection (lower urinary protein excretion) at the end of 24 wk. Severe protein restriction to preserve renal function is a poor strategy for the young rapidly growing individual with chronic renal disease.