Determination of nickel has been carried out by electrothermal atomization atomic absorption spectrometry (ETAAS) using unpyrocoated tubes. A generalized method has been developed for quantitative determination of total nickel in different types of samples: geological, like rock, soil and sediment, environmental, like sewage sludge, and biological, like mussel tissue and rice flour, after their rapid microwave-assisted digestion. The systematic study of the effect of six different matrix modifiers demonstrated that, using an ashing temperature of 1000°C and carrying out the atomization at 2700°C, neither modifier nor background correction are necessaries. The recommended procedure is based on the pressurized acid digestion of samples with aqua regia, HF and H2O2, in different steps, inside hermetically closed teflon reactors, heated by radiation in a microwave oven, at power levels between 330 and 550W. After the complete dissolution of samples, Ni is determined by ETAAS carrying out the absorbance measurements in the peak height mode, and using aqueous solutions of Ni as standards.