Spontaneous rhythmic activity in uterine smooth muscle is maintained by a complex network of smooth muscle cells (SMC), interstitial cells of Cajal (ICC) and a nerve plexus (NP). In rat uterus a spontaneous activity of 1–2 contraction cycles per minutes is regularly observed. The period between the contractions can be prolonged (50%) during application of tetrodotoxin 0.1 mM. The amplitude is nearly unchanged (less 15 %). Using electrical field stimulation, with different current intensities, phasic contractions could be elicited during the silent intervals. Having reached a response of about 60% of the maximum amplitude, the next spontaneous contraction arrived after the normal time interval. Current injection following a response of less than 40% showed a decreased time interval indicating the activation of a pace maker. Systematic change of duration and amplitude of single pulses served for a duration-amplitude-threshold diagram. This relation was unchanged during TTX-treatment. Furthermore, rheobasis and chronaxia were different from those recorded for nerves in gastric smooth muscle (fundus). It is concluded, that uterine pacemakers can be excited by current injection. These pacemakers are less sensitve than nerve cells and their activity is not based on TTX-sensitive Na-influx through voltage gated ion channels.