The relationship between prenatal exposure to methyl mercury and neurologic and developmental abnormalities was ascertained among 234 Cree Indian children aged 12 to 30 months from four northern Quebec communities. A pedlatrlc neurologist, "blinded" to the children's level of exposure, assessed neurologic, physical, mental, and psychosoclal development Methyl mercury exposure was estimated from maternal hair segments representing the period of pregnancy. Abnormality of the tendon reflexes, observed in 13 boys (11 per cent) and in 14 girls (12 per cent), was positively associated with methyl mercury exposure only in boys and there was no consistent dose-response relationship. Other neurologic disorders were less prevalent and none was positively associated with exposure; indeed, incoordinatlon was negatively associated with exposure in girls. The mild, isolated neurologic abnormalities found after prenatal exposure to methyl mercury in northern Quebec were different from the effects of prenatal exposure described in other areas, and their clinical importance can be determined only by continued medical surveillance.