Blood pressure in childhood is an important determinant of hypertension risk later in life, and methylmercury exposure is a potential environmental risk factor. A birth cohort of 1,000 children from the Faroe Islands was examined for prenatal exposure to methylmercury, and at age 7 years, blood pressure, heart rate, and heart rate variability were determined. After adjustment for body weight, diastolic and systolic blood pressure increased by 13.9 mmHg [95% confidence limits (CL) = 7.4, 20.4] and 14.6 mmHg (95% CL = 8.3, 20.8), respectively, when cord blood mercury concentrations increased from 1 to 10 microg/liter cord blood. Above this level, which corresponds to a current exposure limit, no further increase was seen. Birth weight acted as a modifier, with the mercury effect being stronger in children with lower birth weights. In boys, heart rate variability decreased with increasing mercury exposures, particularly from 1 to 10 microg/liter cord blood, at which the variability was reduced by 47% (95% CL = 14%, 68%). These findings suggest that prenatal exposure to methylmercury may affect the development of cardiovascular homeostasis.