Histology of a canine uterus during the first part of diestrus (metestrus) early in the luteal phase. The endometrial glandular epithelium is greatly thickened, glandularity is increased, large quantities of mucous material has accumulated into the lumen of several glands. Areas of normal endometrium are interspaced with areas characterized by the presence of endometrial cysts. This pattern can be defined as physiological cystic endometrial hyperplasia, a phenomenon which develops spontaneously to varying degrees during the progestational phase in the canine and feline uterus and which is generally reversible. However, repeated progestational stimulation (as it occurs with age in the bitch and queen) may lead to areas of endometrium not re-gaining a normal status during the last part of diestrus and anestrus, thus predisposing the bitch to infertility and to developing pyometra. The cellular infiltration in the endometrium suggests that the bitch may have a significant degree of inflammation, a relatively common sequella to a prolonged phase of cystic endometrial hyperlasia. Similar changes can also occur during progestin treatment.