Effect of oestradiol and progesterone on the electromyographic activity (EMG) of the uterus is studied in 6 groups of 2 previously castrated vixens (A, B, C, D, E, F). The animals of gropu A are treated daily with intramuscular injections of oestradiol benzoate alone or oestradiol and progesterone together. These hormones are also given to the group F, oestradiol by a subcutaneous tablet and progesterone intramuscularly. The other females only receive a silastic implant of oestrogen (groups B, C, D, E). Electrical activity is daily recorded during the various treatments, in unanaesthetized animals, by means of platinum electrodes permanently implanted in the uterus. Plasma oestradiol concentrations in peripheral blood are determined in groups A and B. The results show that : 1. OEstradiol induces rhythmic EMG activity composed of bursts of action potentials (Fig. 2). It appears after a delay of 1 to 3 days following the beginning of treatment, except in the animals of group E, treated with a 25 mg oestradiol implant (6 days). Then, EMG activity gradually increases, and the frequency of bursts is at a maximum within 24 to 48 hours. Except in group E, it exceeds 30 per 10 minutes and the amplitude of potentials sometimes reachs 1 mV.