A simple, rapid and reliable method was developed for the determination of total mercury in biological samples. Samples were solubilized using tetramethylammonium hydroxide (TMAH). The organically bound mercury was cleaved and converted to inorganic mercury by on-line addition of KMnO₄. The decomposed mercury together with inorganic mercury originally present in samples was determined by flow injection cold vapour atomic absorption spectrometry after reduction to elemental mercury vapour using NaBH₄. A sample throughput of 100 measurements per hour was achieved after a 30 min dissolution with TMAH. The relative standard deviation for 20 µg l^–1 Hg was 1.3% (n = 11) and the limit of detection was 0.1 µg l^–1 (3σ). The proposed method was validated by the analysis of a suite of certified marine biological reference materials, DORM-2 (dogfish muscle), DOLT-2 (dogfish liver) and TORT-2 (lobster hepatopancreas), with calibration against simple HgII standards.