The water content of the brain is closely related to the maturation and degree of myelination. In control rats studied from the day of birth to the age of 26 days, brain specific gravity increased in parallel to the decrease in water content (r = 1.00). Rats treated with 6-hydroxydopamine (6-OHDA) 100 microns/g daily during the first 2 or 4 days of life had significantly lower specific gravity than controls in all parts of the brain at the age of 2 weeks. With the smaller dose 6-OHDA, the specific gravity at the age of 1 month was lower than in controls in the occipital cortex; with the larger dose it was significantly reduced in all cortical regions as well as in the pons and the diencephalon. 2-months old 6-OHDA treated rats did not differ from controls. Rats pretreated with desipramine 30 min before administration of 6-OHDA to prevent uptake of 6-OHDA into catecholaminergic neurons did not differ from controls. Neonatal 6-OHDA treatment affects predominantly noradrenergic terminals in the cerebral cortex with cell bodies in the locus coeruleus. The present results are consistent with the hypothesis that these neurons have a trophic influence on the brain development during the postnatal maturation.