SUMMARY1. Effects of stimulation of intramural nerves in the circular smooth muscle layer of the body of the oesophagus of the opossum (Didelphis marsupialis) were studied, simultaneously measuring the membrane potential of muscle cells using the sucrosegap technique and contractions of the muscle.2. Electrical field stimulation of the preparation, superfused with Krebs solution at 27 0C, induced a transient hyperpolarization of the smooth muscle cell membrane (i.j.p.) followed by a transient depolarization on which muscle action potentials were often superimposed. The muscle did not develop active tension spontaneously; it therefore did not relax during the i