The motor nerves of the crayfish claw were stimulated by sine waves or by repetitive shocks at various frequencies. The threshold for the opening system remained practically constant up to 1000 c.p.s. The threshold for the closing system was increased considerably by increasing the frequency of stimulation. The threshold in both cases was detd. by mechanograms and by electrograms. The opening system followed the frequencies above 80 c.p.s. by alternation; the closing system reacted irregularly. The velocity of nerve conduction of the opening system is about 3 m. per sec.; that of the closing system about 4 m. per sec. The neuromuscular delay of the opening system in fresh animals varies from 4 to 6 msec; that of the closing system from 3 to 4 msec. The neuromuscular delay of the closing system is always the shorter in a given preparation. The closing system was more susceptible to fatigue than the opening system. It is inferred that the refractory period of the opening system as a whole is shorter than that of the closing system, in splte of its longer neuromuscular delay and slower velocity of conduction.