Sea turtles are endangered species as a result of centuries of overexploitation for meat, eggs and shells, incidental capture and habitat destruction. Moreover, several classes of anthropogenic pollutants discharged into the marine ecosystem could affect their survival. Among the environmental pollutants, heavy metals have a great relevance in ecotoxicology because of their persistence and their possible role as endocrine disruptors. In sea turtles, eggs receive their initial burden with maternal transfer during egg formation and are then exposed to contaminants in the nest environment during incubation. The aim of this study is to provide a risk assessment of heavy metals in the Olive Ridley’s (Lepidochelys olivacea) nesting population in Ostional beach, Costa Rica. From September to December 2012, during and between arribadas events, we are collecting blood samples from nesting females during oviposition to measure trace element concentrations and thyroid hormone levels. In order to evaluate the maternal transfer, we are collecting 2 eggs per sampled female and we will determine thyroid hormones and heavy metals in different eggs fractions (eggshell, yolk and albumen). Following oviposition, exposure to heavy metals in contaminated nest material and soil may also add to chemical burdens in eggs. To assess risk for incubation success and estimate the uptake from the environment, we will also determine the contamination of the nest, by collecting and analyzing sand samples taken from the nest chamber during oviposition. At the time of hatching, we are collecting eggshells and dead embryos, in order to measure heavy metal contamination at the end of incubation, as related to the initial maternal burden. Environmental contaminants can severely affect embryonic development and can potentially contribute to embryo mortality, representing a very important cause of population declines. Ostional beach in northwest Costa Rica is the second most important arribada rookery in the world that supports a large mass-nesting assemblage with persistently low hatching rates. Our study will be the first to determine the level of heavy metal contamination in this population of Olive Ridleys and to test whether there is any correlation with hatching success in Ostional beach.