1. The ventral root dissection technique was used to obtain contractile and electromyogram (e.m.g.) characteristics of ninety-five plantaris motor units in situ in pentobarbitone-anaesthetized rats (n = 20). 2. Motor units demonstrated a wide spectrum of sizes, contractile speeds, and fatigue indices, and were categorized in the same manner as cat hind-limb motor units. Fast-fatigable (f.f.) and fast-intermediate fatigue resistant (f.i.) motor units constituted 20.2 and 25.5% of the motor unit population but together generated over 75% of the cumulative tetanic force. Fast-fatigue resistant (f.r.) and slow motor units composed 43.6 and 10.6% of the population while producing less than 25% of the aggregate tetanic force. 3. Only f.f. and a portion of f.i. motor units demonstrated extensive e.m.g. amplitude reductions during a standard fatigue test. Mean percentage e.m.g. decrease (from the first spike of the first burst to the last spike of the last burst) was 74.0 +/- 27.7% for f.f. units and 28.3 +/- 31.0% (mean +/- S.D.) for f.i. motor units. Relationships between percentage e.m.g. decline and motor unit size (tetanic force) showed significant (P less than 0.01) positive correlations in f.f. (r = 0.71) and f.i. (r = 0.69) motor units. 4. Backward extrapolation of the time course of the force-e.m.g. relationship during the fatigue test revealed that declines in e.m.g. may explain 15, 21 and 66% of the force losses in f.r., f.i. and f.f. motor units. Slow motor units were fatigue resistant and demonstrated a mean e.m.g. decline of 4.3 +/- 6.2%. 5. Indirectly stimulated whole muscle was more fatigable than a composite constructed from motor unit data because of more severe e.m.g. amplitude reductions in the former. 6. The motor unit mechanical and electrical responses during the fatigue test do not summate linearly during whole muscle contractile activity. This is most likely due to the presence, during whole muscle activity, of metabolic changes during the fatigue regimen which influence neuromuscular propagation of excitation, which are not as severe during single motor unit activity.