Localization of metallothionein (MT) in the developing human brain was investigated by immunohistochemical techniques. Fetal brain at 21 weeks showed no MT expression. In 35-week-old fetuses, glial cells in the gray matter showed MT expression in the nucleus and perinuclear cytoplasm, but glial cells in the white matter showed MT not only in the nucleus and perinuclear cytoplasm but also in glial processes. At 40 weeks, glial cells in gray and white matter expressed immuno-reactive MT in the nucleus, cytoplasm and glial processes. Blood vessels were positive for MT. In the infant brain, there were clear differences in glial cells between protoplasmic and fibrous astrocytes; their nuclei, cytoplasm and processes were positive for MT, but the MT-positive glial processes of protoplasmic astrocytes were fine. In the child, typical protoplasmic astrocytes in gray matter and fibrous astrocytes in white matter were observed and their nuclei, cytoplasm and glial processes were positive for MT. Pia mater and blood vessels expressed MT in infants and children.