A membrane filtration procedure for the preconcentration and atomic absorption spectrometric determination of Pb(II), Co(II) and Fe(111) ions in natural water samples has been established. Cellulose nitrate membrane filters (0.45 mum and 47 mm diameter) were used in all experiments. The procedure is based on chelate formation of the analyte metals with 1-(2-pyridylazo) 2-naphtol (PAN) and on retention of the chelates on cellulose nitrate membrane filter. The cellulose nitrate membrane and analyte ions were completely dissolved by 500 muL of nitric acid at 85degreesC on a hood and then metal determinations were performed by flame atomic absorption spectrometry. The method was applied to natural water samples for the determination of analyte ions with satisfactory results, e.g., recoveries > 95%, RSD's < 10%.