1. The uterine response to a single injection of oestradiol-17β during postnatal development of the rat was studied with respect to (i) nuclear binding of oestradiol-17β; (ii) induction of the synthesis of a specific cytoplasmic protein (‘induced protein’ of Gorski); (iii) rate of incorporation of 3 H-labelled amino acids into total protein and into nuclear acid-soluble and acid-insoluble protein; and (iv) rate of [ 3 H]thymidine incorporation into DNA. 2. Specific nuclear binding of oestradiol-17β could be demonstrated even at birth. Administration of oestradiol-17β in vivo caused a significant increase in the number of nuclear binding sites in rats aged 10 days or older. 3. A rapid method is described for the detection of the ‘induced protein’, based on cellulose acetate electrophoresis. Induction of this protein could be demonstrated at the age of 10, 15 and 20 days, but not in 5-day-old rats. 4. In 20-day-old rats the rate of 3 H-labelled amino acid incorporation into protein increased by 3h after oestradiol administration. Incorporation into the different protein fractions reached peak values asynchronously: at 3–4h for acid-insoluble nuclear protein, at 6h for total protein and at about 12h for acid-soluble protein. 5. Treatment with oestradiol failed to stimulate amino acid incorporation into protein in 5- or 10-day-old rats; at the age of 15 to 30 days the hormone caused a significant increase in incorporation into total protein and into both types of nuclear protein. 6. Since the capacity for nuclear binding of oestradiol and for synthesis of the induced protein is demonstrable in the rat uterus before it acquires the ability to respond to the hormone with enhanced general protein synthesis and DNA synthesis, it appears that nuclear binding and the synthesis of the induced protein may be necessary but not sufficient conditions for the trophic action of oestradiol.