A method for the determmatlon of ultra trace concentrations of tnazme herblcldes m natural water usmg silica Cl, solid-phase extraction (SPE) and high-resolution gas chromatography with nitrogen-selectne detection (GC-NPD) has been developed and optlmlzed for estuarme conditions Recoveries of spiked dissolved slmazme and atrazme at a concentration of 100 ng I-' were 82-85% and 87-97%, respectively and were only shghtly affected by the range of sahmtles and pH encountered for estuarme samples The sediment-bound (particulate) tnazmes were ultrasomcally extracted with high recoveries (86-91%) using dlchloromethane The water sample extracts were analysed without any additional clean up whereas the sediment extracts were purified using miniature silica gel columns Nitrogen selective detectlon provided Interference-free detection of slmazme and atrazme m the extracts The detection limits for slmazme and atrazme m water and sediment were 1 ng I-' for 1 I of water and 5 ng g-' 200 mg of suspended sediments The reproduclblhty of the determmatlon of atrazme and slmazme m real water samples at low ng I-' concentrations as shown by %R S D , was better than 10% A comparison of the GC-NPD determmatlon with an alternative method using ion selective detection (GC-ITD) showed very good agreement and indicated superior (ca 5 fold) sensltlvlty of the NPD The method was applied to a study of trmzme herblcldes m estuanne waters of the