In order to examine the relationship between the circadian rhythm in photosensitivity and intermittent light in stimulating testicular development, Japanese quail were exposed to intermittent light providing a single dark period interruption varying from .5 to 10 hr in duration. The treatment lasted for 3 weeks. Changes in the cloacal gland size were monitored throughout the test and testis weights were taken at the end of the test (7 and 11 weeks of age). The results indicated that quail have a circadian rhythm in photosensitivity occurring approximately 12 hr after the onset of light (dark-light interface) which lasts for 4 to 6 hr. Testicular stimulation occurred only when light was given within the photosensitive phase. However, this rhythm is subject to phase shifting by night interrupting light flashes. This resulted in variable testicular responses depending upon the relationship between the position of the new photosensitive phase and the available light. Photostimulation of the testes was proportional to the duration of light within the photosensitive phase up to a maximum of 4 hr of light. Intermittent light may be as effective as a single, long photoperiod in stimulating testes development providing that an adequate amount of light is given during the photosensitive phase.