The nature of the action potential and the mechanical response of crustacean muscle is investigated. If electric shocks of sufficient intensity are applied to the muscle, graded local contractions occur at the cathode. If the intensity of the stimuli is further increased, propagated action potentials, up to 40 mV, are recorded, accompanied by vigorous twitches of the active fibres. The conduction velocity of the muscle impulse is about 20 cm./sec., at 20 degrees C, and its wavelength about 2-3 mm. The mechanical and electrical responses of the muscle to motor nerve stimulation are local or propagated, depending upon the number and frequency of the nerve impulses. With single, or low-frequency, motor nerve impulses a negative potential change is recorded in the vicinity of the nerve endings. It spreads decrementally 2-3 mm. along the muscle fibres, and at 17 degrees C rises to a peak in 3 msec. and falls to one half in about 6 msec. Because of its analogy to the junctional potential of curarized vertebrate muscle it will be referred to as 'end-plate potential' (e.p.p.). The spatial characteristics of the e.p.p. provide evidence for a discrete 'focal' innervation of crustacean muscle fibres, similar to that in vertebrates. In many muscles, with repetitive stimulation, successive e.p.p.'s continue to grow in amplitude for 0.3-0.5 sec. The degree and time course of this 'facilitation' varies greatly in different muscles; depending upon initial size and rate of growth of successive e.p.p.'s, 'fast' and 'slow' systems can be distinguished. At high frequencies (above 100 per sec.), e.p.p.'s sum to a plateau of several times their individual height. When the e.p.p.'s have grown or summed to a 'threshold' level, propagated spikes are set up. Spikes in individual fibres are usually asychronous and occur at a lower rate than e.p.p.'s. If the e.p.p. is slightly below 'threshold', abortive spikes are observed. A prolonged series of e.p.p.'s is associated with a relatively slow maintained contraction of the junctional region. Propagated spikes, on the other hand, are accompanied by quick twitches of the active muscle fibres. This difference is seen clearly by direct inspection of the exposed muscle fibres, but not by recording the overall tension of the muscle. In many muscles, local junctional responses account for more than 50% of the maximum observed tension. Electric recording on the intact animal shows that a good deal of the normal limb muscle activity is based on e.p.p.'s and local contractions. Propagated muscle spikes were seen only during fast and powerful reactions. The rate of contraction varies with the frequency of motor impulses as a higher than second power function. This relation, and especially the origin of the very slow contraction at low frequency, is discussed. Recruitment of individual muscle fibres plays only a minor role; the main factor is the rate of summation of the local mechanical activation process at the junction. A further factor influencing the speed of contraction is the spatial spread of the active region, which controls the extent of internal elastic shortening of the muscle. The various links of the neuro-muscular transmission chain are discussed and compared with the analogous processes in vertebrates.