Neuromuscular junctions in newborn rats differ from adult junctions in several ways, including the number of axonal imputs, the shape of the postsynaptic membrane, and the kinetics of synaptic channels. Neonatal endplates are innervated by 2-4 axons each and have relatively smooth postsynaptic profiles. Adults endplates, however, are singly innervated and have extebsuve secondary membrane folds. in addition, the mean channel open time of subneuralacetylcholine recptors is 3-4 msec in neonates verus 1 msec in adults. Population studies indicate that the loss of excess inputs, membrane folding, and the decrease in channel open time occur at the same time in individual muscles, but it is not known if these changes occur simultaneously in individual fibers. I plan to examine this by studying all three praocesses in the same cells. That is, I will study and endplate physiologically, label it, and study the same endplate in the electron microscope. These experiments will indicate if the decrease in channel open time is directly related to the formation of secondary folds, as has been proposed. They will also indicated if synapse elimination, a widespread phenomenon in developing nervous systems, is cooardinated with maturaltional changes in the target cell. In other experiments, I will determine the roles of nerve and muscle activity in regulating these developmental events. This work is an important step in understanding how a labile, nascent synapse becomes a stabile, mature synapse.