The relationship between serum testosterone level and Hoffmann (H) reflex from right to left thumb flexors was studied in right-handed young men and women. The subjects were divided into two subgroups: fast and slow in right-hand skill, but no difference in left-hand skill (peg moving). The mean serum testosterone level was found to be significantly lower in subjects with fast right hand than those with slow right hand. The mean H reflex from right was significantly smaller in subjects with fast right hand than those with slow right hand. There was a negative, linear relationship between serum testosterone level and amplitude of H-reflex from right thumb. This was more pronounced in females than males. The H reflex from left was not significantly associated with serum testosterone levels. It was concluded that testosterone may affect the somatomotor system of the left brain during adolescence impairing the right-hand skill especially in females. These results also appear to be in accord with the testosterone theory of cerebral lateralization.