The synthesis of nuclear precursors of rRNA and of cytoplasmic mRNA has been estimated in mouse kidneys following contralateral nephrectomy.

1. The production of nuclear rRNA parallels mitotic activity in the proximal tubule cells. Its peak is 2 days after nephrectomy; there may be a second peak at 8 days.

2. The production of mRNA is almost the reciprocal of rRNA production. It is least at 2 days and most at 4 days after nephrectomy. It may also decrease immediately after nephrectomy.

3. Growing kidney cells may build a store of ribosomes before they elaborate mRNA. The sequential relation between rRNA and mRNA synthesis raises the possibility that production of rRNA could regulate the production of mRNA and that the stimulus to compensatory hypertrophy might need to act only long enough to set into motion the machinery for making rRNA, either directly or as a consequence of exhausting mRNA.

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