In the pregnant human uterus, pacemaker activities and their coordination processes were studied in labor induced by uterine distention. Electrical and mechanical activities of the uterus were simultaneously recorded in 6 midtrimester (normal, n = 4; fetal death, n = 2) and 8 term (normal, n = 7; fetal death, n = 1) patients. Initially, frequent and sporadical spike discharges which did not synchronize with low magnitude and irregular mechanical activities were noted. With time, synchronization of both activities developed. These findings were noted regardless of the gestational stage of the patient and fetal viability. The patterns of development and decreases in mechanical and electrical activities, together with their synchronization patterns, were positively and negatively controlled by uterine distention and its suspension, respectively. These results suggest that the uterus has multiple pacemakers in various locations, and uterine distention works as a trigger for the elevation of excitability and for coordinated and orthodromic activities.