During the first 48h of compensatory renal hypertrophy induced by unilateral nephrectomy, RNA content per cell increased by 20-40%. During this period, rates of RNA synthesis derived from the rates of labelling of UTP and RNA after a single injection of [5-(3)H]uridine showed no change in the rate of RNA synthesis (3.1nmol of UTP incorporated into RNA/min per mg of RNA). ATP and ADP pools were not changed. The rate of RNA synthesis was considerably in excess of the increment of total RNA appearing in the kidneys. With [5-(3)H]uridine as label, only continuous infusion for 24h could produce an increase (60%) in the specific radioactivity of renal rRNA in mice with contralateral nephrectomies. With a single injection of [methyl-(3)H]methionine used to identify methyl groups inserted into newly synthesized rRNA, the specific radioactivity of this rRNA was unchanged 5h after contralateral nephrectomy, increased by 60% at 9-48h, and returned to normal values at 120h. Most RNA synthesized in both nephrectomized and sham-nephrectomized mice has a short half-life. Since total cellular RNA content increases in compensatory hypertrophy despite unchanged rates of rRNA synthesis, the accretion of RNA might involve conservation of ribosomal precursor RNA or a change in rate of degradation of mature rRNA.