Previous studies have reported on the propagation of individual spikes in isolated segments of pregnant uterus but there is no information on the patterns of spike propagation in the intact organ. There is also no information on the propagation of the myometrial burst. The aim of this study was to record, at high resolution, the patterns of propagation of electrical activities in the pregnant uterus. Sixteen timed-pregnant guinea pigs were euthanized at term and their uterus isolated. The fetuses were removed and replaced by an equal amount of Tyrode. A 240-electrode array was positioned at various locations along the organ, all signals were recorded simultaneously and the electrical propagations were reconstructed. In the intact pregnant uterus at term, spikes propagated with high velocity in the longitudinal (6.8+/-2.4 cm/sec) and slower in the circular direction (2.8+/-1.0 cm/sec; p<0.01). Direction of propagation and frequency of activity were highly variable but showed similar patterns at the ovary or the cervical end and along the anterior, posterior and the anti-mesometrial border. Along the mesometrium, spike propagation was sparse and fractionated. The migration of the burst (0.6+/-0.4 cm/sec) was significantly much slower than that of individual spikes (p<0.001). Initial burst activity was located at variable locations along the ovarial end of the anti-mesometrial border while latest excitation occurred at the cervical end (1.2+/-0.9 minutes). In conclusion, high resolution electrical mapping of the intact pregnant uterus reveals fundamental properties in spatial and temporal patterns of spike and burst propagation that determine the contraction of the organ.