There are several parameters associated with motoneuron size, among which are the conduction velocity of the axon as well as the size of the excitatory postsynaptic potential (EPSP) induced by stimulation of Ia afferents in the corresponding muscle nerve. In particular, it has been established in animal experiments that small motoneurons with a low conduction velocity exhibit large Ia EPSPs, whereas large motoneurons with a high conduction velocity show small Ia EPSPs. Thus small motoneurons are recruited earlier than large ones. In this study, we investigated whether such a relationship between motoaxon conduction velocity and size of the Ia EPSPs could also be found for human soleus motoneurons. In total, 36 motor units from six healthy volunteers were activated by a slight voluntary contraction and exposed to 200 stimuli of the tibial nerve in the popliteal fossa. Stimuli were delivered using a special stimulus protocol ensuring a constant pre-stimulus spike density along with a constant rate of discharge of the investigated unit. From the stimulus-correlated spike train data a measure of Ia-EPSP amplitude was obtained, along with the single-unit H-reflex latency. Additionally, for each unit, the so-called surface macro EMG was recorded, which measures the complete electrical activity attributable to the unit investigated. From the macro EMB, the intramuscular delay from arrival of each action potential at the soleus muscle and the detection of the muscle-fiber action potential picked up by the recording needle electrode were measured. All single-unit H-reflex latencies were corrected for the corresponding intramuscular delays. From the corrected latencies, single-unit conduction velocities were obtained.(ABSTRACT TRUNCATED AT 250 WORDS)