1. Microelectrode recordings from single motor units of the first dorsal interosseus and the extensor indicis muscles of normal human subjects were studied during voluntary, isometric contractions. The conduction velocity of the nerve fiber innervating the muscle unit was used as an estimator of the size of the motoneuron. 2. During slowly increasing contractions, the units were recruited at force levels which were closely correlated to conduction velocity. The units associated with low conduction velocity were recruited first, those with high conduction velocity, last. 3. If small, stepwise force increments were used instead of slowly, continuously increasing contractions, the units were first activated during the steps and became inactive during the subsequent plateaus. If higher steady-force levels were reached, the activity was maintained also during the plateaus. This steady force, where a unit remained continuously active independent of the rate of rise of tension, represents its tonic threshold. 4. The tonic threshold is positively correlated with conduction velocity, as is the threshold force of recruitment. As a consequence, high-threshold units have a large force range below tonic threshold where they can only be transiently activated, whereas low-threshold units have a large physiological force range above tonic threshold where they operate tonically. The phasic or tonic appearance of discharge pattern reflects quantitative differences in tonic threshold between units of different size. All units examined could be activated phasically (below) and tonically (above tonic threshold). No evidence was found indicating the existence of two qualitatively different classes of units corresponding to a tonic and phasic type, although both muscles investigated consist of about equal numbers of type I and type II muscle fibers. 5. The change in firing rate per unit force was inversely related to conduction velocity: the slower conducting units showed larger changes in firing rate per unit force than faster conducting units. This corresponds to the larger excitability of the smaller units indicated by their earlier recruitment. 6. The data of this study are consistent with the hypothesis that the functional characteristics of human motoneurons are determined by the graded excitability of motoneurons according to size.