For the first time, use of the flooding dose technique using 3H-Phenylalanine is validated for measuring whole-animal and tissue-specific rates of protein synthesis in the blue mussel Mytilus edulis (61 mm shell length; 4.0 g fresh body mass). Following injection, the phenylalanine-specific radioactivities in the gill, mantle and whole-animal free pools were elevated within one hour and remained elevated and stable for up to 6 h following injection of 3H-phenylalanine into the posterior adductor muscle. Incorporation of 3H-phenylalanine into body protein was linear over time following injection and the non-significant intercepts for the regressions suggested incorporation into body protein occurred rapidly after injection. These results validate the technique for measuring rates of protein synthesis in mussels. There were no differences in the calculated rates following 1–6 h incubation in gill, mantle or whole-animal and fractional rates of protein synthesis from the combined time course data were 9.5 ± 0.8% d− 1 for the gill, 2.5 ± 0.3% d− 1 for the mantle and 2.6 ± 0.3% d− 1 for the whole-animal, respectively (mean values ± SEM). The whole-animal absolute rate of protein synthesis was calculated as 18.9 ± 0.6 mg protein day− 1. The use of this technique in measuring one of the major components of maintenance metabolism and growth will provide a valuable and convenient tool in furthering our understanding of the protein metabolism and energetics of this keystone marine invertebrate and its ability to adjust and respond to fluctuations, such as that expected as a result of climate change.