Long days induced LH release in photosensitive Japanese quail and ambient temperature did not affect this process. Temperature also did not affect the levels of circulating LH concentrations at the steady state of LH release on long days. On the other hand, low ambient temperature was required to reduce circulating LH to a nonbreeding level together with the change of the photoperiod from long to short days; the changes of the photoperiod without low ambient temperature induced a decrease of circulating LH only to a certain level (1-2 ng/ml) which could maintain reproductive activity. The results also indicated that there were three levels of circulating LH in quail which were tentatively designated as a nonbreeding level (less than 0.5 ng/ml), a basal breeding level (1-2 ng/ml), and a steady breeding level (ca. 4 ng/ml). By photostimulation, levels of circulating LH increased to the steady breeding level directly and gradually, or leveled off after overshooting it. Photoperiodic changes from long to short days under moderate temperature resulted in a decrease in circulating LH to the basal breeding level which, however, could keep the gonad and the accessory sex organs active. Photoperiodic changes under cold ambient temperature reduce circulating LH to the nonbreeding level at which the gonad and the accessory sex organs regressed completely. These results suggest that ambient temperature is involved in the mechanism controlling an annual reproductive cycle, especially at the termination of the reproductive activity, in Japanese quail.