The anatomy and physiology of the Drosophila larval neuromuscular junction were studied. 2. The dependence of muscle resting potentials on [K+]o and [Na+]o follows the Goldman-Hodgkin-Katz equation (PNa/PK=0-23). Chloride ions distribute passively across the membrane. 3. The mean specific membrane resistance of muscle fibres is 4-3 X 10(3) omega cm2, and the mean specific membrane capacitance is 7-1 muF/cm2. The muscle fibre is virtually isopotential. 4. Transmitter release is quantal. Both the miniature excitatory junctional potential and the evoked release follow the Poisson distribution. 5. Transmitter release depends on approximately the fourth power of [Ca2+]o. If Sr2+ replaces Ca2+, it depends on approximately the fourth power of [Sr2+]o. Mg2+ reduces transmitter release without altering the fourth power dependence on [Ca2+]o.