NADPH-d activity is a marker for neurones which synthesize nitric oxide (NO). In the present paper we study the NADPH-d activity in the human brain stem between 16 and 24 gestational weeks (GW). The adult distribution of NADPH-d containing neurones is established at 24 GW. Between 19 and 21 GW, neurones of the facial, hypoglossal and ambiguus nuclei show transient NADPH-d positivity. Therefore, in the human brain stem there are two chronological patterns of neuronal nitric oxide synthase (NOS) expression, one that appears during prenatal development and persists during the whole life, and another one, present only at a precise foetal moment. The transient NADPH-d activity in motor nuclei suggests that NO is involved in early regulatory processes of the human brain development.