Marine mammals may display high mercury (Hg) levels in their tissues, which raises the question of the importance of toxic metal transfer from mother to offspring. Indeed, Hg could be transferred from mothers to fetuses via the placenta and to suckling pups via the milk, potentially affecting them during their most sensitive periods of development. Some lactating female phocids fast during the suckling period. This fasting period involves not only an important mobilization of energy reserves, but also mobilization of potentially associated contaminants. We studied and compared maternal transfer of Hg to offspring in two phocid species: the northern elephant seal (Mirounga angustirostris) from the Californian coast and the grey seal (Halichoerus grypus) from the Isle of May in Scotland. Total mercury concentrations (THg) were measured in whole blood and maternal milk of 20 mother-pup pairs of each species in early and late lactation. Methylmercury (MeHg) levels were also measured in the blood and milk of grey seals. Results indicated that Hg passed from the maternal tissue into the phocid milk. Milk showed a range of THg levels from 15 to 60 ppb (ng.g-1-wet-weight). From the first days after birth, pups displayed relatively high blood Hg levels suggesting that a Hg transfer through placenta occurred. Blood Hg levels in mothers and pups also varied significantly throughout lactation. While maternal levels doubled, pup levels were reduced by half between the beginning and the end of lactation. Remobilization of proteins and lipids during fasting and milk production in mothers might lead to a release of Hg in blood and therefore increase the levels in late lactation. On the contrary, Hg would be progressively stored in pup organs during their development. Decreasing Hg levels in pups also suggest that the Hg intake via the milk might be lower than that via the placenta. This study highlights a transplacental and transmammary transfer of Hg in both phocid populations. However, further toxicology studies are needed to help understand the potential impact of this Hg transfer. Results also showed that physiological processes such as lactation and/or fasting can modify Hg levels in the blood of mothers and pups. Therefore, such processes and body condition should be considered carefully when interpreting Hg levels in the framework of biomonitoring.