The complex matrix (of different concentrations) effect in the ICP-AES determination of sodium, potassium, magnesium and calcium was studied. Interference due to individual components (NaCl, KCl, CaCO₃, MgCl₂, NH₄H₂PO₄, Hcl, H₂SO₄ and urea) and their concentration levels were evaluated. The developed method was used as a significant part of an automated complex method for the analysis of trace and matrix elements in urine. Easily ionised elements (sodium, potassium) were determined reliably and directly without any sample preparation. Sample dilution was not used because it affected the trace element determination due to their low concentrations and the available detection limits. Suppression of up to 15% of the calcium and magnesium content was found due to the main inorganic components of urine. This effect was overcome by internal standardisation and a matrix-matching technique. The organic components of urine had no significant effect. Concentration levels of Na and K ranged from 100 to 3000 m l⁻¹, Ca from 10 to 300 mg l⁻¹ and Mg from 10 to 200 mg l⁻¹. Precision (RSD) was less than 1% for internal standardisation and less than 2% for direct estimation. Accuracy was evaluated with a urine control material, Seronom™ Trace Element Urine, lot 403125 (Nycomed Pharma).