A current density standard for current shapes used in transcutaneous electrical nerve stimulation (TNS) must be established in order to avoid harmful effects. This is especially important when stimulating near vital structures such as the fetal heart. In the absence of an applicable standard, a preliminary safety norm is proposed, based on clinical experience during delivery and experimental measurements in the female bladder. Current densities due to TNS not exceeding 0.5 microamperemeter/mm2 are safe for the fetal heart. A stimulator and electrodes fulfilling the safety criteria proposed in this study have been tested. A filter which suppresses the electrical disturbances occurring during TNS, thereby permitting recording of the fetal heart rate during birth, has also been tested. TNS was given over both the low-back and suprapubic region. Results of clinical tests of the equipment during 15 supervised births are reported. No adverse effect in the mother or newborn infant were observed.