During replication, human mitochondrial DNA (mtDNA) takes on a triple-stranded structure known as a D-loop, which is implicated in replication and transcription. 1-Methyl-4-phenylpyridinium ion (MPP+), a toxin inducing parkinsonism, inhibits mtDNA replication, possibly by resolving the D-loops. For initiation of mtDNA replication, mitochondria are thought to have another triple-stranded structure, an R-loop. The R-loop, which is resolved by a bacterial junction-specific helicase, RecG, is also resolved by MPP+. Because mitochondrial D-loops are likewise resolved by RecG, the D- and R-loops may share a similar branched structure. MPP+ resolves cruciform DNA in supercoiled DNA. MPP+ converts a stacked conformation to an extended conformation in a synthetic Holliday junction. This conversion is reversed by 1 mM Mg(2+), as is the resolution of the D-loops or cruciform DNA. These observations suggest that the junction structure of mitochondrial D- and R-loops is affected by MPP+.