- Hematological studies concerned with the determination of normal values of blood parameters in animals have been increasing. However, studies on normal concentration of blood constituents of free-living birds still are not very common, and less than 5% of the species of birds have been analyzed, mostly in captivity. Avian hematology has been used in ornithological studies, because it provides biological data about these animals, their biology, and can be very important in the understanding of ecological and behavioral issues. The main purpose of the study was to investigate the concentrations of certain plasma biochemical parameters in nestlings of Rockhopper Penguins (Eudyptes chrysocome) at the creche phase and the potential influence of some factors such as sex. We captured 95 nestling Rockhopper during the period 24-31 January 2017. All nestlings were randomly selected from colonies in Saunders Island (Falkland Islands). All the sampled birds were between 25 and 45days of age, with mean weight of 1.778 +/- 0.314kg and mean bill length of 36.0 +/- 2.8mm. No differences in blood parameters or body condition between sexes were found. No parameters but total protein and urea were related to body index. Body index showed a negative significant relationship with urea levels in blood, with penguins in worse condition (those relatively lighter) showing higher levels of urea in blood than those that were relatively heavier. Same trend was observed for total proteins. Urea concentration in blood would be used as a tool in future studies, particularly in young Rockhoppers when they are in creche phase, a period of high level of mortality mainly by predation. Plasma urea was the single variable that reflects the best body index and also has a rationale background explaining this relationship.