This study investigated the relationship between neonatal testosterone (T) and hand bias in young rhesus monkeys (Macaca mulatta). Subjects (n = 8 per group) included: neonatally androgen-suppressed males, using a Nal-Lys gonadotropin releasing hormone (GnRH) antagonist (Antide); androgen-suppressed males receiving T replacement by a long-acting T preparation (CDB); control males; and control females. Antide suppressed T to the female range, whereas CDB replacement produced supranormal levels. Visually guided reaching, in a social context, showed a population-level left-hand bias. Males with elevated T did not show a stronger left-hand bias than males with normal T, but did show a stronger bias for the preferred hand whether left or right. Males with Antide-suppressed T showed an intermediate degree of hand bias. Results suggest that high neonatal T levels affect laterality and raise the possibility that GnRH analogues influence brain development. These data suggest a broad influence of the CNS-pituitary-testicular axis on brain asymmetries and provide support for an early neonatal period of T-influenced brain differentiation.