Abstract 1. 1. The vitellogenic ovarian follicles of Sarcophaga bullata are surrounded by an asymmetric electric field in S-Ringer. 2. 2. The membrane potential in S-Ringer of the oocyte membrane is − 58.7 ± 1.1 mV and of the trophocyte membrane − 60.8 ± 0.9 mV. 3. 3. In E-Ringer, the extracellular electric field strength is diminished and the polarity is even reversed. 4. 4. Both membrane potential and extracellular electric field can be influenced by the same kind of parameters.