We used the model of one-stage uninephrectomy and contralateral heminephrectomy to study the effect of protein restriction on growth and renal function in young (starting age 21 d) female rats. Normal, sham-operated, pair-fed, and chronic renal insufficiency (CRI) rats were fed a 6, 14 or 22% protein (casein) diet. Diets were otherwise isoenergetic and equivalent in sodium, potassium, chloride, phosphate and calcium. Weight, length, urinary protein and creatinine clearances were measured up to 6 mo of age. Regardless of group (normal, sham or CRI), animals fed a 6% protein diet weighed less and were shorter than animals fed 14 or 22% protein diets. No difference in growth could be found between 14 and 22% protein intake. However, animals with CRI fed 22% protein diet had a shorter survival time than animals fed 14 or 6% protein diets. Six percent protein intake led to a lower growth rate over the first 10-12 wk of age than 14 or 22% protein intake. We conclude that 14% protein intake in rats with CRI maintains normal growth and improves survival over 22% protein intake. Rats with CRI fed a 6% protein diet have improved survival compared to rats fed a 22% protein diet but suffer growth retardation.