Abstract: A microporous glass fiber disk (0.9 cm diameter) was mounted between two plastic layers having a central hole of 0.32 mm diameter. The whole was clamped between two Plexiglass chambers. One of the chambers contained a flow-through electrochemical cell, through which the carrier solution was pumped. A Ag/AgCl reference electrode was immersed in the waste of the carrier solution. The second chamber was equipped with a second Ag/AgCl reference electrode and a potential of 25 mV was applied between the two electrodes. Formation of stabilized BLM was effected by the method of Nikolelis et al. (Ibid.,1995, 7, 531 and Anal. Chem., 1995, 67 936), using solutions of lyophilized egg phosphatidylcholine and dipalmitoylphosphatidic aci d in hexane/ethanol (4:1). The carrier electrolyte solution consisted of 0.1 M KCl/10 mM HEPES buffer of pH 8 containing 1 mM calcium ions. The herbicide solutions were injected into the carrier stream and the resulting transient current signal was recorded. Calibration graphs were linear up to 1.4 ppm, 210 ppb and 300 ppb, respectively, for atrazine, simazine and propazine and the corresponding detection limits were 40, 8 and 20 ppb.