The age-dependent characteristics of compensatory renal growth were studied in neonatal, weanling and young adult rats. Compensatory growth was demonstrated to be greater in young, sexually immature animals than in adults following unilateral nephrectomy. In the four-week interval following surgery in 21-day-old weanling rats, renal mass increased 240% in control animals (group A) and 407% after unilateral nephrectomy (group B). In similarly prepared adult animals, renal weight increased 31 and 81% in the same groups, respectively. The remarkable response in compensatory growth achieved by the young animal was further demonstrated by a greater absolute increase in renal mass in the experimental kidney of young rats compared to adults. Further studies were performed to determine whether the mechanism of compensatory growth in young rats involved formation of new nephrons. The total glomerular count in the remaining kidney of rats after removal of one kidney at 5 (neonatal) and 21 (weanling) days of life did not differ from the value obtained in control neonatal, weanling or adult kidneys. These data indicate that compensatory renal hypertrophy is age-dependent and is greater in young animals. The mechanism for enhanced compensatory renal growth in young animals does not involve glomerular neogenesis since there was no evidence for formation of additional nephrons following nephrectomy either in weanling or neonatal animals.