RATIONALE: Analysis of steroids from precious blubber biopsies obtained from marine mammals, especially endangered species, can provide valuable information on their endocrine status. Challenges with currently used ELISA methodology include lack of absolute quantitation and incompatibility with multiple steroids analysis due to limited biopsy mass. Development of a sensitive, accurate analytical method for this purpose is critical. METHODS: A nanospray liquid chromatography/tandem mass spectrometry (nanoLC/MS/MS) method was validated for sensitive, specific and quantitative analysis of three steroid hormones, without derivatization, extracted from 50 mg blubber samples. Data was acquired with an LTQ XL ion trap mass spectrometer in positive ion mode, using single reaction monitoring. All three steroids were analyzed in a single run. Cholic acid was used as a surrogate internal standard for quantitation due to its steroidal structure and lack of measurable endogenous levels in blubber. RESULTS: The lowest limits of quantitation for progesterone, testosterone, and hydrocortisone were significantly improved compared to previous studies using conventional LC/MS/MS. The lowest limit of detection was 7 fg/μL using a 1 μL injection volume. Calibration curves for steroid quantification showed good linearity (r 2 >0.99) between 14 and 3620 fg/μL, and accuracy was <20% for interday and <10% for intraday. After validation, the method was successfully applied to quantification of steroids in gray whale blubber samples. CONCLUSIONS: The nanoLC/MS/MS method is more sensitive than traditional LC/MS/MS for steroid analysis. It is also compatible with other important biopsy analyses due to its small blubber mass requirement. This will benefit the reproductive and stress assessments for all marine mammals, particularly endangered populations.