It is well known that if ovulation is artificially induced in an immature follicle in the mid-cycle of cows, luteal formation after ovulation is arrested and a hypoplastic corpus luteum is formed. This study was undertaken to investigate if the function of corpus luteum after ovulation is related with the development of ovulating follicle by using FSH. Using 12 cows, 10 or 11 days after spontaneous ovulation, induced ovulation was achieved by administration of 0.8 mg of prostaglandin F₂α analogue (PGF₂α-A) followed by the injection of 200 μg of gonadotrophin releasing hormone analogue 16 hr (groups A and B) or 32 hr (groups C and D) after. A dosage of 10 Armour units (AU) of FSH was administered by intramuscular injections, twice at 16 hr interval, starting either -16 hr in group A, 0 hr in groups B and C or 16 hr in group D after PGF₂α-A treatment. Luteinization after these treatments was checked by rectal palpation, plasma progesterone levels and macroscopic findings at slaughter. Luteal function in groups A and C were improved, compared with groups B and D. Group C showed almost the same plasma progesterone pattern as that of the normal estrous cycle. However, in groups B and D, the improving effect of FSH on luteal formation was slight or poor. In conclusion, it was confirmed that the cause of luteal hypoplasia is closely related to the degree of follicular maturation at the time of ovulation.