Damage to monoaminergic systems during the period of brain development is thought to be involved in several neurodevelopmental disorders. We investigated the maturation of the dopamine and serotonin transporters in rat cerebral regions containing the soma and projections of dopaminergic and serotoninergic neurons in an extensive study from the end of embryonic life (E(18)) to adult stages (until P(70)). The membrane transporters were measured by quantitative autoradiography using specific radioprobes. We demonstrated that the dopamine and serotonin transporters have different patterns of development. The dopamine transporter density increased from E(18) to P(28) where it reached the adult level and then remained stable until P(70). The maturation of serotonin transporters followed a triphasic profile in all areas: (i). an increase leading to a peak obtained between P(0) and P(14) in cell bodies and at P(21) in nerve endings; (ii). a decrease to reach adult levels at P(21) in raphe nuclei and at P(28) in projections areas; and (iii). a plateau until P(70). This demonstrated that the last week of embryonic life and the first two postnatal weeks are critical periods in the development of the dopaminergic and serotoninergic systems at which time they could be particularly vulnerable to injury.