Summary. 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 (>120 sec), long spike bursts (7–120 sec), or short spike bursts (<7 sec) each exhibiting high (>100 μV) or low (50–100 μV) 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 < 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 < 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.
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