Receptors for estrogen and progesterone were measured in cytosols prepared from specimens of canine endometrium obtained at late proestrus, day 4 of estrus, day 2 of diestrus, and at 10 day intervals from days 10 through 80 of diestrus. Twenty nine adult bitches were used, with 2 to 4 dogs used at each time point. Concentrations of estradiol receptors measured in endometrial cytosols from late proestrus through day 10 of diestrus were similar (mean +/- SEM: 9.9 +/- 2.2, 10.5 +/- 1.2, 16.3 +/- 1.6, and 16.2 +/- 2.9 pmol/g of tissue at proestrus, day 4 of estrus, days 2 and 10 of diestrus, respectively). As serum concentrations of progesterone increased during early diestrus, the concentration of estradiol receptors decreased and were significantly (P less than 0.05) lower on days 30 (4.9 +/- 1.3 pmol/g of tissue) and 40 (3.7 +/- 0.6 pmol/g of tissue) of diestrus. After day 40 of diestrus, when serum concentrations of progesterone were approaching basal concentrations, the concentration of estradiol receptors increased and remained significantly (P less than 0.05) higher from days 60 to 80 of diestrus (day 60, 13.4 +/- 2.9; day 70, 15.7 +/- 1.7; day 80, 19.8 +/- 2.4 pmol/g of tissue). As observed for estrogen receptors, the concentration of endometrial receptors for progesterone also gradually increased from late proestrus (4.9 +/- 1.3 pmol/g of tissue) to day 2 of diestrus (6.4 +/- 0.3 pmol/g of tissue).(ABSTRACT TRUNCATED AT 250 WORDS)