The developmental expression of the orphan Na(+)/Cl(-)-dependent transporter, Rxt1, was studied in the rat using a specific [(35)S]complementary RNA probe and affinity purified antibodies. Western blotting experiments allowed the detection of Rxt1 in brain as early as on embryonic day 16. After birth, the brain levels of Rxt1 increased dramatically up to a maximum around postnatal day 30 and then decreased slightly to the adult value. In situ hybridization experiments allowed the earliest detection of Rxt1 messenger RNA in the brain and spinal cord at embryonic day 14. In embryonic day 18 embryos, Rxt1 messenger RNA was present not only in the nervous system but also in the pituitary, the thymus and the heart. Immunoautoradiograms of whole embryo at embryonic days 16 and 18 showed high amounts of the Rxt1 protein in the spinal cord and brain. Moreover, at embryonic day 18, the orphan transporter was expressed in the thymus, heart and liver. At these ages, Rxt1 immunolabeling was localized in neurons of the subplate and in the ventricular zone of the brain. During early postnatal stages, Rxt1 messenger RNA expression demonstrated dynamic and complex changes until postnatal day 13. In particular, this transcript was relatively abundant in the striatum at postnatal days 3 and 5 and then decreased to very low levels after postnatal day 10. At the same period, Rxt1 immunostaining in the hippocampus and the cerebral cortex was observed all over the gray matter, in cell bodies as well as in the neuropil. Finally, the adult pattern was reached around postnatal day 13 for Rxt1 messenger RNA, but only at postnatal day 20 for the Rxt1 protein. The presence of Rxt1 messenger RNA and protein at embryonic stages and the high expression of the protein during synaptogenesis suggest that this vesicular "orphan" transporter is involved in the brain maturation process.