A sensitive, reproducible and rapid method for the spectrophotometric determination of thiocyanate, the metabolite of sodium nitroprusside, was developed. Acetonitrile was used to remove protein in the serum. Uniform design technique was used in arranging the experiments and choosing the reaction conditions between thiocyanate and ferric nitrate reagent. The wavelength of maximum absorption was 456 nm, the calibration curve of SCN- was linear in the range of 1.68-13.4 micrograms/ml (r = 0.9999), the relative average recovery of thiocyanate was 94.0%. The day-to-day and within-day relative standard deviations were less than 4.0% (n = 8) and 3.2% (n = 10) respectively. The limit of determination was 0.3 microgram/ml. Drugs such as lidocaine, etc. presented no interference on the determination.