Radioisotopes are commonly used to study the in vivo metabolism and deposition of dietary fatty acids in adipose tissue. The application of this approach to pinnipeds is problematic because of their large mass and blubber fat content. We have developed a method where labelled lipids can be fed to seals at financially feasible levels, with the radioactivity in individual fatty acids isolated from blubber detected with standard laboratory equipment. A combination of techniques including argentation thin layer chromatography, high performance liquid chromatography with ultraviolet detection, and independent liquid scintillation counting were employed. Juvenile gray seals (Halichoerus grypus) were fed either 0.5 mCi (3)H-labelled triolein (18:1n-9, n=2) or palmitic acid (16:0, n=2). Blubber samples were taken 12 h later, and the radioactivity in individual fatty acids was determined. Radioactivity was detected in only 18:1 from the animals fed (3)H-labelled triolein, indicating direct deposition without modification. Both animals fed (3)H-labelled palmitic acid showed clear peaks of radioactivity in 16:0; however, there was also significant activity (23%-29%) found in the desaturation product 16:1. Our results demonstrate that this method is sufficiently sensitive to track the deposition of labelled dietary lipids as well as modification products of ingested fatty acids and will be important in the application of fatty acid signatures to study predator diets.