1. Cases are given in which female ferrets, already coming into, or in, oestrus as the result of artificially increased lighting in winter and spring, underwent regression or were retarded in their activation by reduction of either intensity or duration of daily illumination.

2. In some cases activation was resumed when light time or intensity was increased again.

3. These results are taken to indicate that, in female ferrets, any inherent rhythm or cycle of sexual activity is modified either naturally or artificially by changes of the cycles of duration and intensity of the light to which they are exposed daily, and so made to correspond in phases with the seasons, or changed therefrom. The rising daily illumination in spring and the falling one in late summer and autumn probably accelerate and retard the sexual activity of the female ferret, or accelerate the onset of anoestrus in late summer and autumn.

4. The data on modification of sexual cycles or activity in ferrets, so far obtained, are consistent with these conclusions.

Aided by grants from the Committee for Research in Problems of Sex of the National Research Council, 1931-33, and the Grants-in-Aid Committee of the National Research Council of the U.S.A., 1931-32; and by laboratory and library facilities of the School of Agriculture and the Zoology Department of the University, Cambridge, England, and the Marine Biological Laboratory, Woods Hole, Mass., U.S.A.; for all of which the author wishes to express his gratitude.