Fast electrical activity of the myometrium was studied in women during pregnancy and childbirth. Action potentials (APs) were recorded from the cervix uteri by means of vacuum electrodes of an original design. APs recorded from the cervix were shown to reflect the electrical activity of the uterus as a whole. Electrohysterography based on recording fast APs can be used to detect changes of tone, even in the absence of visible uterine contractions.