It has been found that the reduction wave associated with dimetridazole in saturated borax solution is enhanced by copper, and that this enhancement is dependent upon the amount of copper in solution. A method is described in which this effect is eliminated.THE polarographic determination of dimetridazole, 1,2-dimethy1-5-nitroimidazole, has been described by Kanel and by Daftsios.2 Kane examined polarographically feed extracts in saturated borax solution from -0.3 to -1.0 volt with a silver-wire anode. He reported that unmedicated feed did not contain any interfering substances but that electro-inactive material depressed the diffusion current by 30 to 35 per cent. Rectilinearity of the diffusioncurrentconcentration relationship was, however, maintained. Recoveries were better than 90 per cent.Daftsios used methanol extracts from feed samples, which, after chromatographic separation, were dissolved in acid solution and examined polarographically from -0-3 to -1.05 volts, with a mercury-pool anode. Chromatographic purification was effective in removing those materials which caused the depression of diffusion currents reported by Kane, and Daftsios reported that the peak heights from samples extracted from medicated feeds were the same as those of pure standards of the same concentration. Polarograms for unmedicated feeds showed no response. Recoveries by the method were good, averaging 98.5 per cent., with a standard deviation of 0.98, on the three feeds tested.In spite of the slightly lower recoveries, Kane's method has the advantage of greater speed and simplicity and this method was chosen for further study by the Prophylactics in Animal Feeds Sub-committee of the Analytical Methods Committee. Three test samples of feed were examined and although agreement was good on two samples, one sample gave consistently high results in certain laboratories. In the authors' laboratories, recoveries of 200 per cent. of dimetridazole were repeatedly obtained with a Davis cathode-ray polarograph. The phenomenon reported by Kane was also observed, i.e., the suppression of peak height in feed extract solutions, compared to the peak height observed in simple aqueous solution. Solutions examined were saturated with borax and determined polarographically from -0-5 to -1-0 volt. The peak potential was -0.85 volt and the peak height in feed solution was 22,000 units compared to a height of 62,000 units for the same concentration of dimetridazole in water.The effect of metal ions on the wave was investigated in an attempt to find the interference that led to high results. By chance, copper was the first element selected for examination and it was found that the element had a marked effect, not only on peak height, but also on the background to the peak. The former was considerably increased and the latter was changed from a flat or, at most, gently rising line to a sharply rising and sometimes stepped slope. The effect of these two interferences on the quantitative determination of dimetridazole in the presence of copper is shown in ...