The applicability of different chemical modifiers for thermal stabilization and ETAAS determination of mercury is studied. The modifier effect is strongly influenced by the type of acid and acid content in the sample solution. A method is described for mercury determination in tuna fish and sediment after high pressure digestion with nitric acid, using thioacetamide (TAC) as chemical modifier. The method permits determination of 0.5 pg g-' mercury. A simpler and faster procedure using a slurry technique for the determination of mercury in reference materials (tuna fish, spinach, cabbage and sediments) was also evaluated. Optimal instrumental parameters for mercury determination in slurries are presented. On the basis of the results obtained a new procedure which allows determination of 0.1 pg g-' mercury was developed.