1. The right kidney in a series of control rats aged between 5 days and 115 days was weighed. The kidney weight/body weight ratio was greater in young than in older rats, but decreased linearly with increasing age.2. After unilateral nephrectomy of rats 5 days old, the remaining kidney underwent compensatory growth. The rate and extent of this growth were greater than in adult rats.3. The concentrations of RNA and DNA in the renal cortex and medulla of rats 5 days old were higher than in adult animals. The concentrations of the two nucleic acids fell with age, and reached adult levels after approximately 6 weeks.4. After unilateral nephrectomy of rats 5 days old, the concentrations of RNA and DNA in the medulla were not significantly different from those in control animals. In the cortex, however, there was a delayed increase in the RNA/DNA ratio, which reached a level some 12% higher than that in control rats. This increase was smaller than that observed in unilaterally nephrectomized adult rats.5. The cortical QO2 of the remaining kidney of unilaterally nephrectomized new-born rats was elevated by some 20% within 1 day of unilateral nephrectomy. Cortical QO2's remained higher than those of control animals for 3-4 weeks.6. Since after unilateral nephrectomy, the increase in renal mass in new-borns was greater than that in adults, whereas the degree of cortical cellular hypertrophy (as estimated by the RNA/DNA ratio) was smaller than in adults, it is likely that in new-born animals a significant contribution to compensatory growth comes from cellular hyperplasia.
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