Summary.In vivo‐investigations of the tonus conditions of the cervical muscles have shown that the tonus is highest in the oestrous phase, lowest in the dioestrous phase and after castration. In substitute experiments it has been shown that the increase of tonus in oestrus is elicited by oestrin. Surviving preparations of circular muscle from the cervix have been studied in different sexual phases. In the oestrous phase the spontaneous activity is of quicker rhythm and greater duration than in the dioestrous. After castration the spontaneous activity tends to cease.Pitocin is without effect on the cervical muscles.Adrenaline causes inhibition and depression of tonus in the cervical muscles.The cervical musculature, in respect of spontaneous activity and tonus, behaves inversely to the rest of the uterine musculature, and in contrast to this is refractory to pitocin but like this is negatively adrenotropic.