The nitroprusside anion, Fe(CN)₅NO^2–, can be accumulated from pH 5 Britton–Robinson buffer solution on to the surface of a hanging mercury drop electrode (HMDE), which has been modified by means of adsorbed poly-L-lysine and which is held on open circuit. This was shown by closing the circuit at –0.2 V versus the Ag–AgCl electrode, after accumulation, and scanning cathodically by use of differential-pulse voltammetry. The voltammetric peak that was observed at about –0.6 V, however, was that for the adsorbed product of the first reduction step of nitroprusside, reported to be Fe(CN)₄NO^2–, which is the product of the one-electron reduction of the nitroprusside ion [to Fe(CN)₅NO^3–] followed by the elimination of CN^–. When accumulation was carried out at the peak potential of the first polarographic reduction peak of nitroprusside (–0.35 V), the peak current at –0.6 V was enhanced 4-fold, which indicated that the Fe(CN)₄NO^2– was accumulated on poly-L-lysine more extensively than was the nitroprusside ion. From pH 2 (0.01 mol dm^–3 hydrochloric acid) solution, nitroprusside can be accumulated very efficiently between 0 and –0.1 V at an unmodified HMDE in the presence of excess of copper(II). The cathodic stripping peak at about –0.35 V is due to the reduction of an adsorbed copper/reduced nitroprusside species, which has not been fully identified, but is most probably either the copper(I) or copper(II) salt of Fe(CN)₄NO^2–. This signal was enhanced slightly when accumulation was effected at a poly-L-lysine-modified HMDE. Linear calibration graphs were obtained for up to at least 1 × 10^–7 mol dm^–3 for the determination of nitroprusside at both pH 5 and pH 2, by using cathodic stripping voltammetric methods based on the above electrode processes, and the limit of detection was calculated to be about 5 × 10^–10 mol dm^–3 in both instances. Relative standard deviations were typically <2% at the 1 × 10^–8 mol dm^–3 level.