The unique relationship between the leader and return stroke in cloud-to-ground flashes and the ambient cloud potential distribution is formulated. This relationship in the form of a ratio of an electric field produced by a leader, ΔEl, to that of a return stroke, ΔEr, is analyzed for two electrostatic leader models, the bi-directional leader and the Schonland ''source charge'' models, and is then compared with field measurements. It is found that the ΔEl/ΔEr ratios of real flashes fall between the curves of the ratios calculated for the two leader models, with each model representing an extreme condition of ambient electric field distribution. ¿American Geophysical Unon 1995.