A comparative examination of reduced methyl [MV·](+) and benzyl [BV·](+) viologens (as artificial electron donors for quantitative estimation of the respiratory periplasmic (Nap) and membrane-embedded (Nar) nitrate reductases) using a newly constructed nap mutant strain of Paracocccus denitrificans was done. The activity with [MV·](+) was high in whole-cell assays, confirming that this compound donates electrons to Nar. Initial rates of the more lipophilic [BV·](+) were considerably lower, which was interpreted to be caused by an inhibition of the active transport of nitrate into the cells. Anionophoric activity of [BV·](+) was detectable but too low to effectively circumvent the inhibition of nitrate transporter.