Two methods are reported for the determination of nickel in zinc plant electrolyte by differential-pulse adsorptive stripping volammetry at a hanging mercury drop electrode, using dimethylglyoxime as the ligand. The first is an on-line method utilizing in situ matrix exchange coupled with a bottom-drain flow-through cell. Use of the matrix-exchange procedure minimizes interferences found from both a variable cobalt concentration and a large excess of zinc in the direct nickel dimethylglyoxime adsorptive stripping method. This on-line method allows for the routine monitoring of nickel concentrations down to 60 µg l^–1 and can be used on a 24 h d^–1 basis as an indicator of levels of nickel which exceed that acceptable for the refining process. In highly pure electrolyte, where nickel concentrations do not affect the refining process, the levels of nickel are too low to be determined by the on-line procedure. However, the determination of nickel could still be required, and a highly sensitive off-line voltammetric method has been developed for this purpose, which employs a nickel dimethylglyoxime solvent-extraction procedure to remove the interference from zinc, followed by adsorptive stripping voltammetry as the dimethylglyoxime complex. The detection limit for this off-line method is 1 µg l^–1, which affords ample sensitivity for quantifying the nickel concentration in highly pure zinc plant electrolyte.