Using [5-3H]orotic acid and [5-3H]uridine as precursors, we compared the efficiency of labeling and the localization of labeled RNA during compensatory hypertrophy of the mouse kidney. [5-3H]orotic acid in tubules labeled RNA 15 times more intensely that [5-3H]uridine, presumably because of greater incorporation of orotic acid into tubular cells. Of the orotic acid label, 97% was in tubular cells, mostly in the proximal tubules. Only about 80% of the uridine label was in the tubules; the ratio in proximal tubules compared with that in distal tubules was 2:1. No changes in distribution within the nephron were produced during compensatory hypertrophy. [5-3H]uridine should be used as the precursor of generalized labeling is desired, but [5-3H5orotic acid is the better precursor of RNA for many studies of compensatory hypertrophy since it is more efficient and concentrates in the segments of greatest biologic activity.