The resting membrane potential of the somatic muscle cells of Ascaris lumbricoides.

1. The resting membrane potential of Ascaris muscle fibres, which is normally relatively insensitive to ion changes in the medium, has been measured under a wide variety of conditions.2. The results have been interpreted in terms of a form of the constant field equation containing additional terms for the contribution of ions and charged groups other than potassium, sodium and chloride.3. Normally the contribution of the additional terms is large and tends to outweigh the contributions of potassium, sodium and chloride.4. The contribution of the additional terms is considerably reduced in the absence of sodium and in the presence of gamma-amino butyric acid and acetylcholine.5. It is suggested that the additional terms may represent the contribution of an electrogenic active transport mechanism to the factors determining the membrane potential.