Using a noninvasive vibrating electrode for the measurement of extracellular current, we show that a polarized ionic current traverses the embryo for many hours in the anuran, perhaps days in the urodele following gastrulation. The voltage driving these ionic currents is an internally positive transepithelial potential (TEP) normally expressed across embryonic integuments. Current is driven out of the lateral walls of the neural folds and the blastopore and enters most of the rest of the embryo's body surface. The magnitude of the TEP is transitorily dependent on external sodium and can be reduced by the embryo's immersion in Na+ depleted media or by treatment with 50 μM amiloride. Both treatments fail to chronically reduce externally detected currents, however. The pattern of currents traversing the embryo suggests they would be associated with rostral-caudal and medial-lateral gradients of voltage within the embryo. By sampling the distribution of TEPs in axolotl embryos, we provide measurements of the former—an internal, caudally negative, potential gradient beneath the neural plate ectoderm. The magnitude of these endogenous fields is on the order of 10 to 20 mV/mm and is within a range of potential known to affect the shape and migration of a variety of embryonic cell types in vitro. We suggest that endogenous currents and voltages in the vertebrate embryo may provide gross cues for cell movement and emerging developmental pattern. © 1994 Wiley-Liss, Inc.