A sensitive flow-injection spectrophotometric method has been developed for the determination of sodium nitroprusside (SNP). It is based on the fact that in alkaline medium, the concentration of the nitrite ions coming from the SNP corresponds with the initial quantity. The nitrite ions are quantified spectrophotometrically by diazotizing with sulphadiazine in acid medium and then coupled with N-(1-naphthyl)ethylenediamine dihydrochloride (NEDA) to give a purple coloured azo dye measured at λmax545 nm. Beer's law is obeyed over the concentration range 0.1-100 μg/ml and the precision (as % RSD) is 1.78% (n=10) for 10 μg/ml. The limit of detection being 0.05 μg/ml (defined as the amount of the drug that gave a signal of twice the back-ground noise). The sample throughput is 17.5 samples h−1. The optimized reagents concentrations as well as the flow injection (FI) parameters were investigated. The applicability of the method to determination of SNP was demonstrated by investigating the effect of potential interferences and by analysis of a commercial pharmaceutical preparation.