Deciduomal reactions in response to artificial stimuli were induced on the tube-ligated uterine horns of unilateral pregnant hamsters. Serum progesterone levels were determined, and the changes of estrogen and progesterone receptors (ER and PR3 respectively) were simultaneously studied in the deciduoma. After stimulation on day 4 of pregnancy (D4), the serum progesterone level rose steadily to about 16 ng/ml on D12 and remained at plateau until D14. A sharp increase followed by a rapid fall in progesterone was observed on D15 and D16. The weight of the deciduoma-bearing horn increased more than 5 times to a maximal level on D10. Both deciduomal and myometrial weights were maintained and remained elevated on D16. Histological observation revealed that the maintenance of decidual cells was heterogenous. Cells at antimesometrial site (AMS) started to regress on D10, whereas cells at mesometrial site (MS) remained recognizable on D16.Cytosol ER was detected during the early stage (D6) of decidualization. The cytosol ER in the deciduoma continuously decreased and became undetectable on D10. Nuclear ER was not detected during the time of study. The nuclear and cytosol PR increased and reached a peak level on D8 and D10) respectively. The concentration of PR declined sharply afterward. The PR levels in deciduomal tissues at MS and AMS were separately studied. PR in AMS decreased gradually after D8 to a basal level on D14. PR at MS still remained relatively constant on D12 followed by a precipitous decrease. These data show a prolongation of the deciduomal life span and the heterogeneity of deciduomal maintenance and regression in unilateral pregnant hamsters. This strongly suggests a close relationship between the loss of PR and the regression of the deciduoma.