A new, simple and rapid spectrophotometric FI method for the accurate and precise determination of Hg(II) in pharmaceutical preparations has been developed. The method is based on the measuring the decrease of absorbance intensity of p-nitrobenzoxosulfamate (NBS) due to the complexation with Hg(II). The absorption peak of the NBS, which is decreased linearly by addition of Hg(II), occurs at 430 nm in 2x10(-4) moll(-1) HNO(3) as a carrier solution. Optimization of chemical and FI variables has been made. A micro column consisting of several packing materials applied instead of reaction coil was also investigated. A background level of Fe(III) maintained in reagent carrier solution with NBS was found useful for sensitivity and selectivity. Under the optimized conditions, the sampling rate was over 100 h(-1), the calibration curve obtained were linear over the range 1-10 mugml(-1), the detection limit was lower than 0.2 mugml(-1) for a 20 mul injection volume, and the precision [S(r)=1% at 2 mugml(-1) Hg(II) (n=10)] was found quite satisfactory. Application of the method to the analysis of Hg(II) in pharmaceutical preparations resulted a good agreement between the expected and found values.