The force-velocity relations of single glycerinated rabbit psoas muscle fibers at 5 degrees C were studied at maximum and half-maximum activation in the presence of 0 (control) and 39-145 g/liter dextran T-70. Resting fiber diameter decreased progressively to approximately 70% of the nondextran control as the dextran concentration was increased. Isometric force at full activation increased to a maximum of 136% of control at 111 g/liter dextran and then fell to 80% of control in 145 g/liter dextran. Maximum velocity, which fell to 49% of the control value in the highest concentration of dextran, was nearly constant at approximately 65% control over the range of 58-111 g/liter dextran. Relative maximum power, which gives an estimate of changes in intermediate velocity, was not significantly reduced by dextran concentrations up to 76 g/liter, but then fell progressively to 62% of control in the highest concentration of dextran. At half-maximum activation, maximum velocity and relative maximum power were not significantly different from the values at full activation. The results obtained at partial activation indicate that the decline of velocity seen in the presence of dextran is not due to a passive internal load and that the dextran does not cause a viscous resistance to shortening. The increased velocity in the absence of dextran can be explained by the reduced ability of cross-bridges to resist shortening, as proposed by Goldman (1987. Biophys. J. 51:57).