Hibernation does not correspond to a period of rest of the ovary, nor to the prolongation of a period of anoestrus. Hibernation is necessary for the normal course of the period of genital activity. Its suppression tends to modify more or less severely the physiological equilibrium implicated in the control of genital activities. However, the animal can find a more or less normal rythm even if is deprived of hibernation during the course of a second year. Prolongation of hibernation in hemicastrated animals during the entire period of genital activity suppresses the ovular discharge and the ovulating secretion of FSH and LH, without affecting the tonic secretions of FSH. It inhibits the phenomenon of compensatory hypertrophy. However, in hemicastrated animals deprived of hibernation, the reaction to hemicastration does not take place nor during anoestrus nor during mid-december or mid-march but only when the period of genital activity begins again.