To determine whether nitro group reduction occurs in mammalian tissues, metronidazole (0.021, 0.064 and 10 mg/kg), misonidazole (0.015 mg/kg) and nitrofurazone (0.13 mg/kg), respectively, were administered to germfree rats. A reduced metabolite [1-(2-aminoimidazol-1-yl)-3-methoxypropan-2-ol] and two of its hydrolysis products, urea and (2-hydroxy-3-methoxypropyl)-guanidine, were found in the urines of germfree rats that received misonidazole. When nitrofurazone was administered, a reduced metabolite, 4-cyano-2-oxobutyraldehyde semicarbazone, was detected in the urines. However, acetamide and N-(2-hydroxyethyl)oxamic acid, fragmentation products from the reduction of metronidazole, were not found in significant concentrations in the urine when germfree rats received metronidazole. Apparently metronidazole is reduced so much more slowly than misonidazole and nitrofurazone in the tissues of germfree rats that its reductive metabolites are not detectable. This observation may be explained by the one-electron reduction potentials (E1 7) of these drugs, that of metronidazole (E1 7 = -486 mV) being lower than those of either misonidazole (E1 7 = -389 mV) or nitrofurazone (E1 7 = -257 mV). Under these circumstances, metronidazole reduction is not detected, either because its radical anion forms more slowly than that of the other nitroheterocyclic compounds or because its radical anion interacts more rapidly with oxygen to restore the parent compound.