The role of orthophosphate ions (Pi) in crossbridge kinetics was investigated by parallel measurements of the ATP hydrolysis rate and tension transients in maximally activated, chemically skinned rabbit psoas fibers. The hydrolysis rate of the standard activation at 20 degrees C was measured at 1.25 nmole X s-1 X m-1 X fiber-1, which corresponds to the hydrolysis of 3 moles ATP per mole of myosin head per second. The isometric tension, stiffness extrapolated to the infinite frequency, and the ATPase rate progressively decreased when increasing concentrations of Pi (0-16 mM) were added to the activating saline. The decrease was greatest with tension, followed by stiffness and the ATPase rate. Both the apparent rate constant and the magnitude parameters of exponential process (B) increased with Pi concentration resulting in a significant increase in the oscillatory power output. The effects of Pi on processes (A) and (C) were only marginal. When fibers were oscillated at 1 Hz [close to the characteristic frequency of process (A)], no significant increase in the ATP hydrolysis rate was observed. However, a small increase was noticed at 10 Hz [1%, process (B)], and at 100 Hz [6%, process (C)]. We interpret these results in terms of a crossbridge scheme which adds a branch pathway to the conventional hydrolysis cycle. In the proposed scheme, the number of crossbridges entering the branch pathway increases at higher Pi concentrations and in the presence of imposed oscillations at the proper frequency.