Abstract A diet containing 0·6 ppm of a mixture of polychlorinated dibenzofurans (PCDFs) having 4, 5 or 6 chlorine atoms was fed to mice for 18 days after mating or for 14 days after delivery. Dams, foetuses and offspring were analysed for PCDFs by gas chromatography. PCDFs were transferred to the foetuses across the placenta and to the offspring through milk. The amounts of PCDFs transferred through milk were much larger than the amounts transferred across the placenta. A greater level of PCDFs was accumulated in the livers of the dams than in other tissues. There were differences between the PCDFs in their levels of accumulation in the tissues of the mice.