All embryos that have been investigated drive ionic currents through themselves and these currents will generate internal electric fields. Here, those examples in which such fields have been measured directly are discussed. The first such measurements were made in chick embryos and about 20 mV mm(-1) was measured near the posterior intestinal portal in 2-4 day-old embryos. This electric field is important for the development of tail structures because reducing its magnitude results in abnormal tail development. The second embryonic electric field measured directly was in the axolotl, where a rostral-caudal field of about the same magnitude was detected. Modification of this field during neurulation but not gastrulation caused developmental abnormalities. Most recently, the development of left-right asymmetry in frog and chick embryos was found to require a voltage difference between blastomeres at a very early developmental stage. This field was measured in the chick embryo to be 10-20 mV mm(-1) across the primitive streak. Mammalian skin wounds generate 150 mV mm(-1) fields lateral to the wound and corneal epidermal wounds exhibit lateral fields of 40 mV mm(-1). The presence of these endogenous fields would suggest that exposures to external electric fields should be limited to magnitudes of less than 0.1 V m(-1).