It is known that the properties of the excitation-contraction coupling of crayfish skeletal muscle are different in some respects from those of frog muscle. In the present study, activation of contraction of the crayfish muscle induced by short depolarizing pulses was investigated and it was compared with the results of frog muscle obtained by Adrian, Chandler and Hodgkin (1969). Two glass microelectrodes were inserted into the thoracal muscle of the crayfish. The muscle was stimulated by the voltage-clamped pulses of different durations and the resulting contractions were observed under the binocular microscope with the magnification of 60 X at 20 approximately 23 degrees C. The rheobasic membrane potential was -55mV. The mechanical threshold potential was -42 mV for 10 msec, -15 mV for 2.5 msec, +18 mV for 1 msec and around +90 mV for 0.5 msec pulses. For short pulses where the threshold potential was more positive than -20 mV, the area of the depolarization above -30 mV was 51 mV-msec. Subthreshold pulses produced contraction if applied repetitively. The effect of a just suprathreshold short pulse on the activation of contraction was cancelled by the hyperpolarizing pulse.