The luminal epithelium of the uterus has been examined morphometrically in ovariectomized rats from 0--25 hours after a single injection of either 2.5 mg progesterone or 2.5 micrograms oestradiol subcutaneously. In each case, the epithelial cells were 50% taller at 25 hours than in controls. In progesterone series, this growth occurred between 6 and 25 hours: it was preceded by a significant increase in nuclear size and in the volume fraction of cytoplasm occupied by free ribosomes at 3 hours, and increases in the volume fractions of free ribosomes and RER at 6 hours. AT 25 hours, many smooth-surfaced vesicles appeared at the cell apex. In the oestradiol series, cell size decreased at 3 and 6 hours before increasing at 25 hours. Apical vesicles were reduced in number at all times after oestradiol: at 25 hours the nucleolus showed a fivefold increase in size, the RER had a greater volume fraction, and the lipid droplets were significantly reduced in number. The nature and timing of these results are consistent with the hypothesis that progesterone acts on these cells by a different mechanism to oestradiol, which does not involve DNA-directed RNA synthesis.