Syrian Golden hamster dams were administered ²⁰³Hg-labelled methyl mercury (MeHg; 1.6 μmol/kg) 1 day after parturition and milk was collected twice during the 1st week. The excretion of ²⁰³Hg in milk and the uptake, retention and tissue distribution of ²⁰³Hg in the pups was studied using gamma counting. The fraction of inorganic Hg in milk and in the kidneys of the pups was determined following separation of inorganic Hg and MeHg by ion exchange chromatography. The concentration of ²⁰³Hg in milk on the 1st day after MeHg administration was 0.12 nmol/g. ²⁰³Hg was mainly (80–90%) excreted as MeHg during the first 6 days of lactation. The whole body and tissue concentration of ²⁰³Hg in the pups increased for 10–15 days and decreased thereafter. The content of ²⁰³Hg in the pelt and the fraction of inorganic Hg in the kidney increased throughout the study period (4 weeks). The excretion of MeHg in milk corresponded to at least 5% of the dose administered to the dam. Our study demonstrates that breast milk may be a significant source of MeHg exposure during the critical neonatal period.