Skeletal muscles consist of motor units which may differ considerably in contractile properties and types of usage. Some units participate mainly in relatively rare, quick movements and contract rapidly and are easily fatigued (type FF); others contribute to the maintenance of posture and hence contract slowly and are fatigue-resistant (type S), while others are both fast and fatigue-resistant (type FR; ref. 1). Our understanding of motor control mechanisms and the dependence of contractile properties on usage would be enhanced if more quantitative information were available^2–4 concerning the firing patterns of individual motor units during normal motor behaviour. Therefore, we have made continuous recordings for extended periods from single motor units in the fast extensor digitorum longus (edl) and the slow soleus (sol) muscle of freely moving adult rats. By counting the total number of discharges for each unit, and by determining the distributions of interspike intervals and the duration of the individual impulse trains, we have obtained information about firing rate, amount of use, modulation of muscle force and tonic and phasic behaviour for 16 motor units. We now report that these units fall into three classes apparently corresponding to type FF and FR in the edl muscle and type S in the soleus muscle.