The effects of the long-acting prostagrandin F-2alpha-analogue fenprostalene were investigated. Twenty-three female beagles (24 cases) 1-6 years of age were divided into 6 groups, and a dose of 5-150 mug/head of fenprostalene was subcutaneously administered at 25 days after ovulation to investigate its effects based on peripheral blood progesterone (P-4) levels. The dogs were also examined for shortening of their estrous cycle after administration and fecundity after the recurrence of estrus. The results showed that the administration of 50 mug/head of fenprostalene or more reduced the levels of peripheral blood P-4 to about 1 ng/ml 2 days after administration, indicating early regression of the corpus luteum, and that the administration of these doses shortened the time to the subsequent estrus by a mean of about 80 days, and that conception rates were normal if estrus recurred about 2 months after fenprostalene administration or later.