The present study was conducted to determine the effects of PGF₂α and dexamethasone on corpus luteum in pregnant rats. PGF₂α and dexamethasone were injected separately and simultaneously subcutaneously twice a day for 4 days beginning on day 10 of pregnancy. The plasma progesterone levels were measured following each injections. Also the ovarian, uterine, luteal and adrenal gland weights, number of corpora lutea, as well as number and size of implants were investigated. Ovaries and adrenals were examined by light microscopy. The results obtained were summarized as follows: 1. Treatment with PGF₂α or dexamethasone separately or in combination reduced significantly the plasma progesterone levels to 81.4 ± 0.7, 27.3 ± 0.6 and 28.6 ± 0.6 ng/㎖ respectively, compared to 135.7 ± 0.7 ng/㎖ in the control rats (p$lt;0.01). 2. The weights of ovaries and corpora lutea and the number of corpus luteum were slightly decreased as compared to the control group but the difference was not statistically significant in the groups. 3. The uterine weights and the number and size of implants were significantly decreased as compared to the control group (p$lt;0.01). They were decreased markedly in the two groups of rats receiving PGF₂α treatment alone and PGF₂α/dexamethasone combined as compared to those of rats treated with dexamethasone only. 4. The adrenal gland weights were not affected by treatment with PFG₂α or dexamethasone alone or in combination. 5. In the histological findings of adrenals of the dexamethasone treated rats, there was an atrophy of the adrenal cortex, hyperemia, hemorrhage, vacuolization and necrosis of the cortical cells. 6. In the histological findings of ovaries for the PGF₂αtreated rats, pyknotic nuclei and cytoplasrnic vacuolization of luteal cells were observed. In the dexamethasone treated rats, congestion and hemorrhage, varuolization and necrosis of luteal cells and necrosis of granulosa cells were observed. Also the corpus luteum contained necrotic foci.