We previously reported the presence of transiently tyrosine hydroxylase (TH)-only-immunoreactive (ir), non-catecholaminergic (non-CAnergic) neurons in some brain regions of postnatal mice; anterior olfactory nucleus, medial geniculate nucleus, and spinal trigeminal nucleus, where CAnergic terminals but not cell bodies are present in the adult wild mouse. These transiently TH-ir brain regions of the postnatal wild mouse showed stable TH-immunoreactivity in the adult brain of the human TH transgenic (hTHTg) mice. TH expression was also observed in the nucleus parabigeminalis of the hTHTg mice. Using the antiserum against GTP cyclohydrolase I (GCH), first rate-limiting enzyme of the biosynthesis of tetrahydrobiopterin (BH4), the cofactor for TH, we proved that these TH-only-ir neurons in the wild mice and in the hTHTg mice were not stained with the antiserum against GCH. The results indicate that these TH-only-ir neurons which do not synthesize the BH4 cofactor do not produce dihydroxyphenylalanine, suggesting a new unknown function of TH in these neurons.