The relationships among nonverbal intelligence, hand speed, and serum testosterone level were studied in male left-handers ranging in age from 17 to 19 years. Hand speed was measured by a peg moving task. To assess the differences between nonverbal IQs. Cattell's Culture Fair Intelligence Test was used. There was a direct correlation between IQ and testosterone. IQ increased linearly with right-hand speed, which was directly related to testosterone. There was no significant correlation between IQ and left-hand speed, which was not significantly correlated with testosterone. IQ decreased with left- minus right-hand speed, which also decreased with testosterone. It was suggested that nonverbal spatial reasoning ability may be directly associated with the efficiency of left brain, which is favored by testosterone in male left-handers. It was also concluded that the left to right asymmetry in hand speed may depend on efficiency of the right brain in left-handed males.