Rate constants have been obtained using oxygen isotope exchange techniques for steps controlling ATP release and Pi binding in the ATPase cycle of insect flight muscle fibres from the giant waterbug Lethecerus. The new exchange data for Pi binding and ATP release are compatible with a model developed previously in which only the rate constants controlling Pi and ATP release change during fibre activation. Phosphate-water oxygen exchange occurs into ATP remaining after partial hydrolysis by chemically skinned fibres in (18O) water. For fully activated fibres, the results are compatible with a single set of rate constants controlling this exchange and give a rate constant for ATP release of 1 s-1 (21 degrees C, pH 7.0 I = 120 mM). Oxygen exchange also occurs between (18O4)Pi in the medium and water during ATP hydrolysis. There is a strong correlation between the measured rate constant of exchange and the value of keat for the ATPase activity at different levels of activation. For fibres fully activated by oscillation or strain, the rate constant for Pi binding to an actomyosin. ADP state is greater than 960 M-1 s-1.