Measuring the quality of the nutritional resources available to wild herbivores is critical to understanding trophic regulation processes. However, the direct assessment of dietary nutritional characteristics is usually difficult, which hampers monitoring nutritional constraints in natural populations. The feeding ecology of ruminant herbivores has been often assessed by analyzing fecal nitrogen (FN) concentrations, although this method has been less evaluated in other taxa. This study analyzed the suitability of FN as an indicator of ingesta quality in the European rabbit, which is a keystone lagomorph species in Mediterranean ecosystems and of great conservation interest. Domestic rabbits were used under experimental conditions, Near-Infrared Spectroscopy (NIRS) techniques, and a pilot field study was conducted in three different habitats during wet and drought periods. A strong association was found between diet type and total FN and metabolic FN. It was also found that NIRS calibrations were accurate for depicting nitrogen concentrations. Finally, the seasonal FN dynamics measured in the field were consistent with current knowledge on vegetation dynamics and forage limitations in the three habitats. The results support the use of NIRS methods and FN indices as a reliable and affordable approach to monitoring the nutritional quality of rabbit habitats. Potential applications include the assessment of the mechanistic relationships between resource limitations and population abundance, e.g., in relation to natural drought cycles and to habitat interventions aimed at reinforcing rabbit populations. informacion[at]ebd.csic.es Gil-Jiménez (2015) Fecal Nitrogen Concentration as a Nutritional Quality Indicator for European Rabbit Ecological Studies. PLoS ONE 10(4): e0125190. doi:10.1371/journal.pone.0125190