1. Glycerol-extracted flight muscle from the giant water-bug, Lethocerus cordofanus, undergoes contraction when deprived of ATP. This rigor contraction occurs in the presence of the calcium chelating agent EGTA, under conditions in which the predicted free calcium ion concentration is less than 5 x 10(-9)M. A mechanically similar contraction has been observed by depriving the muscle of Mg(2+), in the presence of 5 mM-ATP.2. In the rigor contraction tensions of up to 120 mN/mm(2) under isometric conditions, and shortenings of between 2(1/2) and 6% under isotonic conditions, have been observed at 20 degrees C.3. Muscles in rigor can be relaxed by the addition of ATP. The minimum concentration of ATP required to give full relaxation depends upon other ionic constituents of the solutions, and upon temperature, but is between 0.3 and 1 mM at 20 degrees C. Lower concentrations result in partial reduction of tension and stiffness.4. Pyrophosphate (PP) causes a reduction in the tension of muscle which has developed rigor under isometric conditions, but the stiffness, when measured at frequencies between 1 and 100 Hz, remains indistinguishable from that of rigor muscle. The stiffness when measured by applying slow length changes is comparable to that of ATP-relaxed muscle. It is suggested that in PP-relaxed muscle the cross-bridges remain in close proximity to, but are not rigidly attached to, the I filaments, resulting in a high viscous interaction between the two sets of filaments.5. The addition of low concentrations of ADP (less than about 0.7 mM) to rigor muscle in the absence of ATP causes an increase in tension. The effect is sigmoid, and is probably due to the formation of low concentrations of ATP throughout the body of the fibre by myokinase activity. Larger concentrations of ADP (about 5 mM) added to rigor muscle cause relaxation, probably due to the formation of higher ATP concentrations.6. AMP and inorganic orthophosphate have little effect upon the mechanical properties of rigor muscle.7. There is a delay of about 1 min before the onset of rigor contraction when fibres are transferred from an ATP-solution to one containing no ATP, due to the transfer of ATP in the body of the fibres. Both ATP hydrolysis by the fibres and diffusion of ATP into the bathing solution contribute significantly to the rate of depletion of ATP.8. Rabbit psoas muscle shows a similar rigor contraction in the presence of EGTA, and has mechanical properties similar to those described for Lethocerus flight muscle in the presence of pyrophosphate.