We have used a vibrating probe and intracellular recording techniques to study the development of a steady electric current generated by rat lumbrical muscle. In adult animals, previous work has revealed a steady outward current generated at the end plate region. In the present study, we show that at birth muscles generate a steady inward, not outward current. The inward current declines with age, disappearing about 5 days after birth. At about the same time, the steady outward current appears, and reaches adult amplitude by 2-3 weeks after birth. The two currents are generated by completely different mechanisms. The inward current is blocked by alpha-bungarotoxin and apparently results from activation of acetylcholine-gated channels at the end plate. The outward current, on the other hand, is not affected by alpha-bungarotoxin but is blocked by agents which interfere with chloride movements across the membrane, as in the adult.