A so-called "night-interruption" experiment with a 15-min light pulse showed that a sensitive phase for the photoperiodic LH secretion in male Japanese quail extended over a period of 2 hr from 12.5 to 14.5 hr after dawn. Exposure of a 1-hr light pulse at this photosensitive phase to male quail kept under 8L:16D induced the increase of plasma LH concentrations just the same as quail transferred to 16L:8D. In the first few days of photostimulation either by night interruption or by long days, LH concentrations increased at several hours after the photosensitive phase and decreased to the basal levels before day-break. The amplitude and duration of this LH surge was somewhat like a preovulatory LH surge in females. However barbiturate anesthesia (pentobarbital and phenobarbital) administered on, before, or after the photosensitive phase did not block the LH increase by photostimulation. On the other hand, an injection of phenobarbital 14 hr before the expected ovulation blocked a preovulatory LH surge, even though the same drug failed to block photoinduced LH increase in females. These results indicate that the neuroendocrine mechanism involved in photostimulated LH release is different from that for an LH surge during an ovulatory cycle.