The electrochemical reduction of 1-[(2-hydroxyethyl)chloro-methyl]-2-methyl-5-nitro-1H-imidazole (ornidazole) was studied by normal pulse polarography and reverse pulse polarography. In aqueous solution with no surfactant, ornidazole gives rise to one wave (I) or two waves (I and II) depending on pH. The electrode reactions corresponding to these waves were characterized, and appropriate reaction mechanisms are hypothesized. In basic solutions containing DMF or sodium lauryl sulphate, wave I splits into two or three. When splitting is complete, the electrode reaction corresponding to the first daughter wave, I′, is the simple, reversible formation of the anion radical. This circumstance, and the analogous behaviour of metronidazole and tinidazole, was used to determine the formal potential of the nitroimidazole/anion radical system for ornidazole, metronidazole and tinidazole in aqueous media; the results agreed remarkably well with previously reported values obtained by pulse radiolysis. Optimal conditions for the analytical determination of ornidazole by differential pulse polarography were established.