In the grazing heifers with quiescent ovaries, the responses of their ovaries to administration of gonadotropins (Experiment I) and of an LH-RH analogue (Experiment II) were investigated with the check of fertility following the treatments.<BR>1. In Experiment I, 13 animals averaging 18.9 months of age and 299 kg in weight received an intramuscular (i.m.) injection of PMS, 1, 000 IU and HCG, 1, 000 MU. Ovulation was induced in all animals by 35 days after the treatment. The size of corpora lutea formed following the induced ovulation seemed likely normal in 5 animals, whereas those of the remaining 8 animals were a little smaller. Corpora lutea in 12 animals regressed in rather sooner or within 812 days after the treatment, and in one animal, it regressed in the period of the normal estrous cycle length. After regression of these corpora lutea, ovaries of 7 animals (53.9%) showed normal cyclic activity, but in remaining 6 animals, their ovaries returned quiescent state again. Two out of 4 animals received artificial insemination (A. I.) on 2736 days after the treatment, became pregant.<BR>2. In Experiment II, 26 animals averaging 18.5 months of age and 301 kg in weight were alloted to the two groups; i.e. group 1 (13 animals) received an i. m. injection of 200 μg of [Des-Gly-NH¹⁰₂, Pro-ethylamide9]-LH-RH (LH-RH-A), and group 2 (13 animals) received an i.m. injection of 400 μg of LH-RH-A. Ovulation was induced by 24 days after the treatment in ten and seven animals for groups 1 and 2, respectively. Corpora lutea formed following the induced ovulation were rather small in size or hardly noticeable by rectal palpation, and they regressed in rather sooner or within 612 days after the treatment. The ovaries of 4 animals (30.8%) each in both groups showed normal cyclic activity after regression of their corpora lutea, but those in the remaining animals (total 18 in both groups) returned quiescent state again. The ovaries of the 9 animals which had not ovulated by the 4th day after the treatment, did not show and cyclic activity for the following 36 days (8 animals), or in one animal, it turned to be cystic in this period. Three out of 5 animals received A. I. on 934 days after the treatment, became pregnant.<BR>3. The hematocrit value of blood collected at the time of treatment, tended to be lower in the animals with non-responded ovaries (including one which became cystic) than in the animals with ovaries which responded and were activated by the treatment. The result may indicate that the response of quiescent ovaries of the heifers to the hormones administered in this study seemed to depend upon the difference of the physiological condition of each animal, at the treatment given.