The purpose of this experiment was to investigate the effects of hysterectomy and administration of PGF₂α on serum progesterone level and corpus luteum in pesudopregnant rats. PGF₂α administration or hysterectomy was performed at day S of pseudopregnancy. Five rats from each group were randomly sacrificed at day 5,6,7,8,9 and 12 of pseudopregnancy for assay of serum progesterone level and microscopic observation of ovary. The results obtained were as follows; 1. In both normal and hysterectomized groups, PGF₂α administration caused significant reduction in serum progesterone level to 24.2% and 30.8% at day 6 and to 11.3% and 34.3% at day 9 of pseudopregnancy as compared with pretreatment level. The serum progesterone level of hysterectomized group untreated with PGF₂α remained elevated until day 12 of pseudopregnancy as compared to those of other three groups. 2. The ovarian weights of control group treated with PGF₂α were significantly reduced as compared to those of other three groups at day 9 of pseudopregnancy (p$lt;0.05). 3. The number of corpus luteum in groups treated with PGF₂α was slightly reduced, but there were no significant difference between PGF₂α and non-PGF₂α treated groups at day 6,7,8,9 of pseudopregnancy. 4. The number of lutein cell in corpus luteum of PGF₂α treated groups on day 9 of pseudopregnancy was significantly lower than those of PGF₂α untreated groups (p$lt;0.05).