The dependency of the rat estrous cycle on the daily alternation of light and dark.

pendency of the estrous cycle on the light rhythm was investigated with special reference to the age. The lighting condition utilized was the normal lighting, the continuous lighting and continuous complete darkness. For the last condition, the special boxes were designed to feed the animals under complete light-proof condition. Two kind of estrous cycles were distinguished; one was dependent on the diurnal rhythm of light and dark (the"light dependent cycle"), and the other was independent of the rhythm (the"light independent cycle"). The former showed regular 4-day cycles, but in the latter each animal was repeating nearly 4-day cycles independently on chronological day and night. The"light dependent cycle" was seen in the animals which were provided the normal lighting from birth, and if these animals were transfPrred into the continuous lighting, the cycle was disturbed and the constant estrus resulted in about 15 days. This constant lighting stimuli, however, did not exert any detectable effects on the animals which had active corpora lutea. The "light independent cycle" was seen in the animals brought up without providing any