Spinal motor asymmetry was studied in relation to handedness. Hand preference was assessed by Oldfield's Questionnaire and Geschwind scores. hand skill was evaluated by a peg moving task. The motoneuronal excitability was assessed by the size and recovery curve of the H-reflex elicited by stimulation of the right and left median nerves. H-reflex was recorded by cup electrodes placed over the wrist flexors. The mean reflex latencies from the right and left sides were 23.4 and 23.2 ms, respectively. The mean amplitude of the maximum H reflex was significantly higher on the right side than the left side in right-handers without familial sinistrality (FS). There were no significant side differences in the amplitudes of H-reflexes of the right-handers with FS. A genetic factor was suggested to involve this left shift of spinal motor lateralization in right-handers with FS. The recovery curve studies showed that the motoneuronal excitability was higher on the right than the left in right-handers without FS. There was no excitability difference between the right and left sides of the right-handers with FS. In left-handers, the motoneuronal excitability was significantly higher on the left than the right side. Voluntary flexions of the wrist increased the H-reflex especially on the right side of the right-handers. There was no relationship between this corticospinal facilitation and baseline EMG activity. The H-reflex amplitude was found to be inversely correlated with hand skill in right-handers. It was concluded that motoneuronal excitability is associated with handedness and also depends on FS. It was suggested that small reflexes are especially suitable in fine motor control of rapid aimed-movements, as the well-established relation between the size of motor units and fine motor control.