With electromyography and strain gauges the uterine motility of the sow during the oestrous cycle and early pregnancy was studied. Special attention was paid to characterization of myometrial activity at the time of intra-uterine migration of blastocysts. From recordings of 4 animals (3 became pregnant) 3 types of electrical spiking activity (each could be correlated with an elevation of the strain gauge tension) were discerned. Two of them appeared regularly and were suitable for analysis: "High Voltage Slow Acticity" (with relative high amplitude and low frequency of spikes) and "Low Voltage Fast Activity" (with relative low aplitude and high frequency of spikes). The sexual status markedly influenced myometrial activity: during proestrus and oestrus only one type of electrical activity was present whereas the myoelectrical complex (an episode of this activity and the subsequent interval of inactivity) was shorter than 10 min. During dioestrus the myoelectrical complex was longer than 10 min and "High Voltage Slow Activity" (solely on the cervix and bifurcation) and "Low Voltage Fast Activity" occurred simultaneously in episodes which mainly appeared to originate on the bifurcation. The characteristics of uterine activity during pregnancy were similar to those of a cyclic sow until day 12. It was only on day 12 that gestation appeared from an increased frequency of myoelectrical complexes. It is concluded that "Low Voltage Fast Activity" as it was found on the uterine horn at days 8-9 might be involved in the process of intra-uterine migration of blastocysts. In cyclic and in pregnant animals the patterns of "Low Voltage Fast Activity" were similar. Therefore, the occurrence of "Low Voltage Fast Activity" is independent of the actual presence of blastocysts. It seems to be exhibited in dependence on the ovarian hormones.