Uterine lipid components of mice were analysed during different stages of the oestrous cycle and after ovariectomy and replacement therapy. During oestrus, and in ovariectomized animals subjected to oestrogen treatment, there was a fall in total lipid concentration and percentage of triglycerides, but an increase in concentration of phospholipids, as compared to the corresponding di-oestrous values. Progesterone antagonized the effect of oestrogen and restored the triglyceride concentration to the di-oestrous level. In comparison, the effect of progesterone on total lipids or phospholipids was less marked. There was no significant change in uterine cholesterol concentration during different stages of the oestrous cycle or after ovariectomy and hormone therapy. In general, the fatty acid components of the lipid constituents showed lower iodine values and increased saturation under the influence of oestrogen. These changes were more evident in triglyceride and phospholipid fractions than in the cholesterol ester fraction. Of the nitrogenous bases of phosphoglycerides, serine showed a more marked increase than the other components. The possible significance of these findings is discussed.