Luteinizing hormone (LH) and sex steroid hormones in the pituitary, plasma, and hierarchical follicles of the quail maintained under 14L:10D condition were measured during the ovulatory cycle at 2-hr intervals using radioimmunoassay. Pituitary levels of LH during the ovulatory cycle showed two peaks which occurred 16 and 10 hr before ovulation. A significant decrease in LH concentration was observed from 10 to 6 hr before ovulation, shortly after the onset of light (dawn). Plasma LH started to increase from 10 hr before ovulation, and reached a peak 6 hr before ovulation. Plasma progesterone also started to increase shortly after dawn and reached a peak 4 hr before ovulation. A single peak of plasma testosterone was observed 6 hr before ovulation. Two peaks of plasma estradiol concentration were observed at 22 and 6 hr before ovulation. A similar tendency of the change in plasma concentration of estrone to that of estradiol was observed during the ovulatory cycle. In the largest follicle, the progesterone content started to increase from 10 hr before ovulation, reached a peak 2 hr before ovulation and decreased abruptly after the ovulation. The contents of the second to fourth largest follicles were low. These findings strongly suggest that progesterone in the largest follicle is a main cause of ovulation by producing enzymes which act to break the follicle membrane. The estradiol contents of the third and fourth largest follicles were high, and showed the peak at 18 hr before ovulation in the third largest one and at 8 hr before ovulation in the fourth largest one. The contents of the largest, ruptured, and the second largest follicles were low compared with the third or fourth largest one. These results suggest that the onset of light may be a signal for the increases of plasma LH and progesterone and follicular progesterone which are responsible for the induction of ovulation in the quail.