Uptake and depuration kinetics of dissolved 203Hg and 210Pb were determined during the entire embryonic development of the eggs of the cuttlefish, Sepia officinalis (50 d at 17°C). 203Hg and 210Pb were accumulated continuously by the eggs all along the development time reaching load/concentration ratio (LCR) of 467 ± 43 and 1301 ± 126 g, respectively. During the first month, most of the 203Hg and 210Pb remained associated with the eggshell indicating that the latter acted as an efficient shield against metal penetration. From this time onwards, 203Hg accumulated in the embryo, indicating that it passed through the eggshell, whereas 210Pb did not cross the chorion during the whole exposure time. It also demonstrated that translocation of Hg associated with the inner layers of the eggshell is a significant source of exposure for the embryo. This study highlighted that the maturing embryo could be subjected to the toxic effects of Hg in the coastal waters where the embryonic development is taking place.