Myoelectrical activity in the uterus of late-pregnant sheep was recorded by the chronic implantation of bipolar electrodes in the myometrium. Three distinct patterns of electrical activity were recognizable: irregular trains of action potentials (greater than 120 sec), long spike bursts (7--120 sec), or short spike bursts (less than 7 sec) each exhibiting high (greater than 100 muV) or low (50--100 muV) amplitude. The duration of spike activity was expressed as the myoelectrical index (MI), i.e. the fraction of the hour during which a given category of activity is exhibited. The increase in the overall MI which represents daily average values regardless of the class of amplitude from 72 h pre partum up to parturition was significantly (P less than 0.05) more pronounced in the uterine horn than in the body of the uterus. During the last 6--7 days of gestation high-amplitude irregular trains and long spike bursts were significantly (P less than 0.05) more frequent than these activity patterns with low amplitude. Regardless of the amplitude, the incidence of irregular trains began to decline rapidly 48 h pre partum with an abrupt cessation following parturition. The frequency of long spike bursts increased 72 h pre partum to reach a maximum which coincided with parturition.