A very simple, fast method for the spectrophotometric determination of sodium nitroprusside based on the application of kinetic methodology and the batch mixing technique to the coupling reaction between the diazotized nitroprusside derivative and N-(1-naphthyl)ethylenediamine dihydrochloride is proposed. The high initial rate of this reaction allows analytical measurements to be made within only 21 s which makes the method applicable to automatic routine analyses. The calibration graph is linear over the range 0.1–100 μg ml−1and the precision (as % RSD) is less than 1.5%. The proposed method was satisfactorily used for direct analysis of pharmaceutical formulations containing sodium nitroprusside.