Because of the toxicity and mobility of organic mercury, there is a need for determination of organic and inorganic mercury at very low concentrations in the environment, especially in seawater. A procedure for extraction and determination of low concentrations of the different states of mercury - total, inorganic, and organic - in seawater, by cold vapour atomic absorption spectroscopy (CVAAS) is described. Inorganic mercury only was directly reduced to the Hg(0) state by tin(II) chloride under strongly acid conditions (H(2)SO(4)) and mercury metal (Hg(0)) was determined by cold vapour atomic absorption spectroscopy. Organic and inorganic mercury were extracted, with toluene, as the bromide derivatives and re-extracted, together, into ammonium chloride solution. Organic mercury was converted into inorganic mercury by thermal digestion at 80-90 degrees C in the presence of strong oxidants. These two states of mercury were determined together as total mercury. Inorganic mercury was measured directly after pre-concentration of the sample by toluene extraction. Toluene dissolved in aqueous phase after re-extraction of the sample was removed by heating for 30 min at 80-90 degrees C. Organic mercury was calculated as the difference between total and inorganic mercury. The sensitivity of the method is 0.0001 ng mL(-1) Hg, depending on sample volume.