An attempt to reproduce pyometra by means of experimental uterine infection was conducted with the 10 mongrel hitches of the anestrus, proestrus, metestrus, and postpartum uterine involution stages. Organism used was hemoltic E. coli. origially isolated from uterine pus of a bitch that had the canine pyometra. One ml of broth culture containing $2{ imes}10^6$ CFU was inoculated into the uterine horn with the uterine cervix ligated. Aか bitches were ovariohysterectomized at the 7th or 14th day after inoculation and the uterus was provided for macroscopic and microscopic observations. An experimental production of pyometra was successful in 7(70%) of 10 mongrel bitches, and the incidence of pyometra was seen 100% in the stage of anestrus, 100% in the stage of metestrus, 50.0% in the stage of pyomestrus, and 33.359 in the stage of uterine involution. Histologically, these uteri were characterized by the presence of cystic. endomeetrial hyperplasia(CEH) and acute endometritis superimposed on the cystic hyperplasia. And the pyometra with CEH could be seen 66.7% in metestrus 50% in anestrus, 33.3% in involution and in 0% in proestrus stage. Blood progesterone concentrations were showed as a normal range of the each stage of sexual cycle and any relationships between the progesterone concentrations and the incidence of pyometra were not observed.