The role of phosphate ions (Pi) in crossbridge kinetics is investigated by parallel measurements of the ATP hydrolysis rate and the tension transients in maximally activated, chemically skinned rabbit psoas fibers. The tension transients are induced by sinusoidal length oscillations, and resulting time courses are analyzed in terms of three exponential processes (A), (B), and (C). The ATP hydrolysis rate is measured by the NADH fluorscence method coupled with enzymatic reactions which rephosphorylate ADP and oxidize NADH. The hydrolysis rate of the standard activation at 20 degrees C is measured at 0.61 mM/sec. The isometric tension, stiffness, and the ATPase rate progressively decrease with increasing concentrations of Pi (0-16 mM). The decrease is most notably observed with tension, followed by stiffness and the ATPase rate. Both the apparent rate constant and the magnitude parameters of exponential process (B) increase with Pi concentration resulting in a large increase in the oscillatory power output. The active species of Pi which causes this effect is found to be the monovalent anion H2PO4-. The effects of Pi on processes (A) and (C) are only marginal. When fibers are oscillated at 1 Hz, no increase in the ATP hydrolysis rate is observed; a small increase is noticed at 10 Hz (1%), and at 100 Hz (6%). We interpret these results in terms of a crossbridge scheme which adds a branch pathway to the conventional hydrolysis cycle: the number of crossbridges entering the branch pathway increases at higher Pi concentrations and in the presence of oscillations.