Currently, the mechanism of metronidazole resistance is not understood in Helicobacter pylori. We have looked at uptake of metronidazole into a sensitive and a resistant strain of H. pylori. Both strains displayed rapid uptake of [14C]metronidazole, although the resistant strain accumulated the drug at a slower rate and to a lesser amount than the sensitive strain. Uptake was inhibited by KCN and carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) but not by sodium arsenate. Thin-layer chromatography analysis of lysed cell supernatants showed that metronidazole was metabolized in both strains. A variety of related imidazole compounds inhibited metronidazole uptake, consistent with a common transport system for this group of antibiotics. Our data do not support an absence of uptake or metabolism as a cause of resistance in the strain examined.