We have imposed steady DC electric fields over developing gastrula and neurula stage axolotls, Ambystoma mexicanum. These applied voltages were meant to confound cues provided by endogenous currents and fields that we have measured and believe to be controls of pattern and morphogenesis during early development. Applied voltages in the physiological range (25–75 mV/mm) cause severe disruption of development when imposed over neurulae whose orientation within the field is random or fixed. In the latter case, developmental defects are more likely to occur at that end of the neurula (rostral or caudal) that faces the cathode, or negative pole, of the applied field. In neurulae whose orientation within the field was fixed, the lowest magnitude producing developmental abnormality was between 5 and 25 mV/mm. Physiological measurement of the embryonic transepithelial potential (TEP) when perturbed by artificially applied voltages demonstrates that ectoderm facing the cathode is hyperpolarized, while ectoderm facing the anode is depolarized at all fields strengths tested. These data show that the electrical polarity of embryonic ectoderm is predictably disrupted by applied voltages. Though applied voltages exert this same effect on the ectoderm of gastrulae, exposure only during gastrulation does not lead to developmental abnormality. This observation demonstrates that the applied electric field does not harm the embryo in some non-specific way and further emphasizes the stages of neurulation as those most sensitive to artificially applied or endogenous voltages. These data strongly support the notion the polarized natural voltages within amphibian embryos are controls of their emerging pattern. © 1994 Wiley-Liss, Inc.