1. The influence of inorganic phosphate (P(i)) on the relationship between ATP consumption and mechanical performance under isometric and dynamic conditions was investigated in chemically skinned single fibres or thin bundles from rabbit psoas muscle. Myofibrillar ATPase activity was measured photometrically by enzymatic coupling of the regeneration of ATP to the oxidation of NADH. NADH absorbance at 340 nm was determined inside a miniature (4 microliters) measuring chamber. 2. ATP consumption due to isovelocity shortenings was measured in the range between 0.0625 and 1 L0 s-1(L0: fibre length previous to shortening, corresponding to a sarcomere length of 2.64 microns), in solutions without added P(i) and with 30 mM P(i). To get an estimate of the amount of ATP utilized during the shortening phase, quick releases of various amplitudes were also performed. 3. After quick releases, sufficiently large that force dropped to zero, extra ATP was hydrolysed which was largely independent of the amplitude of the release and of the period of unloaded shortening. This extra amount, above the isometric ATP turnover, corresponded to about 0.7 and 0.5 ATP molecules per myosin head at 0 and 30 mM P(i), respectively. 4. ATP turnover during the isovelocity shortenings was higher than isometric turnover and increased with increasing shortening velocity up to about 2.7 times the isometric value. At low and moderate velocities of shortening (< 0.5 L0 s-1), P(i) reduced ATP turnover during isovelocity shortening and isometric ATP turnover to a similar extent, i.e. a decrease to about 77% between 0 and 30 mM added P(i). 5. The extra ATP turnover above the isometric value, resulting from isovelocity shortenings studied at different speeds, was proportional to the power output of the preparation, both in the absence and presence of added [P(i)]. 6. The effect of shortening velocity and [P(i)] on energy turnover can be understood in a cross-bridge model that consists of a detached, a non- or low-force-producing, and a force-producing state. In this model, mass action of P(i) influences the equilibrium between the force-producing and the non-or-low-force-producing cross-bridges, and shortening enhances cross-bridge detachment from both attached states.