Concentrations of calcium and phosphorus were measured in the neonatal dentine of 11 crabeater and 11 Weddell seal postcanine teeth with an energy-dispersive x-ray analyser. The extent of variation in elemental concentrations in different parts of the tooth, differences between species and individuals, and whether variation in elemental concentrations can provide information about dentine deposition mechanisms were assessed. No consistent patterns in elemental deposition in different parts of the tooth were found, but there were differences in concentrations between and within species. Post-natal dentine is composed of layers that appear alternately bright and dark in backscattered electron images. The elemental composition of neonatal dentine was closer to the dark bands than to those that appeared bright. It is suggested that the composition of neonatal dentine is more similar to the dark than the bright layers of dentine because of nutritional stresses that were occurring during mineral deposition.