In the Japanese quail, ovulatory responses of the largest follicle to LH, P₄ or synthetic LHRH administered at various times during an ovulatory cycle were studied. Each of the hormones at several dosages was injected 24, 21, 18, 15, or 12h before the predicted time of Cs ovulation. Premature ovulation occurred when oLH was injected during the earlier period of an ovulatory cycle (i.e. 21 and 18h before ovulation). In contrast, P₄ given during the same period failed to induce premature ovulation. The discrepancy between LH and P₄ on the ovulation-inducing effect fround during the ealier period of an ovulatory cycle might be due to a low responsiveness to P₄ of hypothalamo-pituitary axis. In addition, it was also found that the largest follicle could not ovulate by LHRH injected during the same period. These results may suggest that the pituitary gland remains in a low responsiveness to LHRH and P₄ on the release of LH until 18h before Cs ovulation while the largest follicle has already acquired a high ovulability in response to LH at the same time.