Because the ability to accelerate rapidly is crucial to the survival and reproductive fitness of most terrestrial animals, it is important to understand how the biomechanics of rapid acceleration differs from that of steady-state locomotion. Here we compare rapid acceleration with high-speed galloping in dogs to investigate the ways in which body and limb posture and ground forces are altered to produce effective acceleration. Seven dogs were videotaped at 250 Hz as they performed ;maximum effort' accelerations, starting in a standing position on a force plate and one and two strides before it. These dogs began accelerations by rapidly flexing their ankles and knees as they dropped into a crouch. The crouched posture was maintained in the first accelerating stride such that the ankle and knee were significantly more flexed than during steady high-speed galloping. The hindlimb was also significantly more retracted over the first stance period than during high-speed galloping. Ground forces differed from steady-state locomotion in that rapidly accelerating dogs supported only 43% of their body weight with the forelimbs, compared with 56-64% in steady-state locomotion. The hindlimbs applied greater peak accelerating forces than the forelimbs, but the forelimbs contributed significantly to the dogs' acceleration by producing 43% of the total propulsive impulse. Kinematically, rapid acceleration differs from steady-state galloping in that the limbs are more flexed and more retracted, while the back undergoes greater pitching movement. Ground reaction forces also differ significantly from steady-state galloping in that almost no decelerating forces are applied while propulsive force impulses are three to six times greater.