The relation of canine mammary tumor to concentrations of estrogen and progesterone receptors has not been clarified yet, although such female sex hormones should be suspected of affecting the development of mammary tumors. Using two groups composed of 23 dogs with mammary tumors, we measured the concentrations of estrogen and progesterone receptors in the tumor tissues in each dog, and compared the values with those from five healthy dogs. The severity of the dog's diseases was classified lnto stages I to V according to the classification of MacEwen et al. Out of 23 dogs with mammary cancers, eight dogs at stage I had 7.76 fmol/mg protein and 11.30 fmol/mg protein as their mean values of estrogen and progesterone receptor concentrations; six dogs at stage II had 8.54 fmol/mg protein and 10.37 fmol/mg protein; seven dogs at stage III had 5.61 and 10.06; one dog at stage IV had 4.57 and 9.14; and the remaining one at stage V had 1.97 and 5.07. Out of 23 dogs with malignant mammary mixed tumors, 20 dogs at stage I had 9.06 fmol/mg protein and 16.27 fmol/mg protein; two dogs at stage II had 3.04 and 15.23; and the remaining one at stage III had 1.97 and 4.44, In the other words, estrogen and progesterone receptor concentrations tended to decrease as the disease progressed in both groups. This tendency was more marked in estrogen receptor concentrations, although this is inconclusive because the number of the dogs at advanced stages was too small in this study.