Dopamine is an important neurotransmitter in the human central nervous system and also plays a key role in the development of postnatal brains. We previously reported that nicotinamide, a SIRT1 inhibitor, regulates tyrosine hydroxylase (TH) expression in vitro. To investigate the effect of nicotinamide-mediated TH regulation in vivo, nicotinamide was chronically injected into neonatal mice. Interestingly, nicotinamide-treated mice were smaller in size, and their locomotor activity was reduced. L-DOPA treatment caused hypersensitive locomotor activity that indicates a dopamine-depleted state. These changes seemed to be associated with dopamine metabolism in hypothalamus, since dopamine in hypothalamus was reduced but not in striatum. The present study suggests that the regulation of dopamine metabolism during the postnatal development is important and the underlying molecular mechanisms may be associated with SIRT1 signaling.