An on-line lead preconcentration and determination system implemented with inductively coupled plasma atomic emission spectrometry (ICP-AES) combined with a flow injection (FI) method with ultrasonic nebulization (USN) was studied. The lead was retained as the lead-diethyldithiocarbamate complex at pH 9.5. The lead complex was eluted from the knotted reactor (KR) with 4.0 mol l ^–1 hydrochloric acid. A total enhancement factor of 140 was obtained with respect to ICP-AES using pneumatic nebulization ( 14.8 for USN and 9.5 for KR). The detection limit for the preconcentration of 10 ml of aqueous solution was 0.2 ng ml ^–1 . The precision for ten replicate determinations at the 20 µg l ^–1 lead level was 2.7% relative standard deviation, calculated with the peak heights obtained. The calibration graph using the preconcentration system for lead was linear with a correlation coefficient of 0.9993 at levels near the detection limits up to at least 100 ng ml ^–1 . The method was successfully applied to the determination of lead in tap water samples.