Administration of methylmercury (1 or 2.5 mg/kg daily) to neonatal rats caused alterations in both cardiac and renal growth patterns. Heart weights were elevated in the preweaning period in association with hyperplasia (supranormal DNA content); after weaning, the cardiac overgrowth regressed and there was an eventual hypoplasia as evidenced by low DNA content in young adulthood. Renal overgrowth was more pronounced and persistent, but reflected a pure hypertrophy, with no changes in DNA. Despite the absence of apparent morphologic damage to the kidney, renal function was affected by neonatal methylmercury exposure, as assessed through basal clearance techniques. In the immediate period after beginning treatment, there was an impairment of renal function (elevated serum urea, creatinine and osmolality; increased fractional excretions of water, sodium and osmotic particles), with a return to normal by 10 days postnatally.