Most intracloud discharges are supposed to be initiated by hydrometeors in a vertical ambient electric field. However, horizontal propagating flashes are commonly observed. The aim of this work is to measure the horizontal onset field of instability for various drop sizes and to compare the results with those previously determined in experiments where the drops did not reach their terminal velocity <Nolan, 1926; Macky, 1931; Ausman and Brook, 1967> and with those recently published by Kamra et al. <1993> for drops suspended in a wind tunnel. In the present experiment, uncharged drops fall about 14 m and are at terminal velocity before entering an interelectrodes region where a horizontal electric field is generated. It appears that the lowest onset field intensities are obtained when the aerodynamical effects are maximum. However, there is a large discrepancy between our results and those by Kamra et al. It is believed that the onset fields of instability are underestimated by the latter authors because of probably unperfect experimental conditions. The present results, which are certainly more representative of the fall of uncharged drops in still air than those previously obtained, seem to indicate that horizontal electric fields in thunderclouds are not strong enough to cause lightning initiation by corona emission in these conditions and to affect the drop-size distributions. This conclusion conflicts with that of Kamra et al. <1993>. However, the case of charged drops falling at higher altitude would certainly lead to lower horizontal field onsets. ¿ 1997 American Geophysical Union