Measurements of electric fields associated with thunderstorms and lightning were obtained during two balloon flights carrying double-probe electric field detectors launched from Cachoeira Paulista (22 ¿44'S, 44 ¿56'W), Brazil, on January 26, 1994, and March 23, 1995. From data obtained in 1994, a linear relationship between the quasi-dc vertical electric field peak amplitude and the decay time constant of lightning signatures was found for negative flashes. The results are compared to similar data for intracloud flashes. Based on electric field data obtained in 1995 and on the present knowledge about the differences between positive cloud-to-ground and intracloud flashes, two methods to distinguish them at balloon altitudes are presented: The first is based on an estimate of the destroyed charge in the event; the second is based on the peak amplitude ratio between the vertical quasi-dc and the VLF electric field. The behavior of the vertical quasi-dc electric field before and after large cloud-to-ground lightning flashes is discussed and attributed to the existence of a shielding layer around the thunderstorm. This shielding layer is associated with a threefold or greater decrease in the conductivity inside the cloud. An abrupt variation observed in the quasi-dc electric field possibly associated with the occurrence of positive flashes was observed and attributed to the formation of a transient shielding layer just above the thunderstorm, which could be produced by the near-breakdown field inside the cloud.