The gait of the adult Swiss (Mike Flack--MF1 subtype) mouse during spontaneous walk/trot locomotion at velocities ranging from 14-43 cm s(-1) has been analysed using simultaneous video and reaction force analysis. No differences were observed between males and females. Velocity adjustments within this range are accounted for to a greater extent (>70%) by stride time decreases and to a lesser degree (<30%) by stride length increases. Equivalent stride times for fore and hindlimbs were, in the former, composed of a shorter stance and a longer swing time. Peak vertical reaction force increases with decreasing stance time, with that for the forelimb being about 5% greater than that for the hindlimb across the whole stance time range studied. The areas under the vertical reaction force curves for fore and hindlimbs are, however, not significantly different. The results are discussed in the light of in vitro work cycle studies on the properties of some of the major hindlimb locomotor mouse muscles, and with previously established data in the rat. It is concluded that the mouse shows a consistent and quantifiable gait that would allow incorporation of locomotor assessment into the evaluation of a number of pathophysiological states.