Electric activity of the uterus was recorded by 6 chronically implanted wire electrodes in 17 unrestrained 5-day cycling rats. Results obtained during 196 h of recording revealed consistent changes in frequency, amplitude, temporal pattern and in direction and distance of propagation of electric activity. In estrus, bursts were short and of variable amplitude and frequency, while in metestrus bursts had high amplitude, longer duration and regular frequency. The activity decreased from metestrus to the first diestrous day and still more to the second diestrous day. In diestrus and proestrus long bursts appeared once to twice within an hour. In proestrus the morning level of activity was still low, but high at night, when it resembled the activity in estrus. Electric activity spread in both directions but with a higher frequency in the cervical direction in all phases of the cycle. Cervical electric activity appeared in synchrony with that of the uterine body and did not differ from it in type.