The vector electric field and polar conductivities were measured by zero-pressure balloon-borne payloads launched from Wallops Island, Virginia during the summers of 1987 and 1988. Data were collected over thunderstorms (or electrified clouds) during 6-hour flights at altitudes near 30 km. The vector electric field measurements were made with the double Langmuir probe high-impedance method, and the direct conductivity measurements were obtained with the relaxation technique. We show evidence for conductivity variations over thunderstorms (or electrified clouds). We find that both positive and negative polar conductivity data do show variations of up to a factor of 2 from ambient values associated with the disturbed periods. Some ideas for possible physical mechanisms which may be responsible for the conductivity variations over thunderstorms are also discussed in this paper. ¿ American Geophysical Union 1989