It has long been established that the inner membrane of plant mitochondria is permeable to Cl-. Evidence has also accumulated which suggests that a number of other anions such as Pi and dicarboxylates can also be transported electrophoretically. In this paper, we present evidence that anion uniport in plant mitochondria is mediated via a pH-regulated channel related to the so-called inner membrane anion channel (IMAC) of animal mitochondria. Like IMAC, the channel in potato mitochondria transports a wide variety of anions including NO3-, Cl-, ferrocyanide, 1,2,3-benzene-tricarboxylate, malonate, Pi, alpha-ketoglutarate, malate, adipate, and glucuronate. In the presence of nigericin, anion uniport is sensitive to the medium pH (pIC50 = 7.60, Hill coefficient = 2). In the absence of nigericin, transport rates are much lower and much less sensitive to pH, suggesting that matrix H+ inhibit anion uniport. This conclusion is supported by measurements of H+ flux which reveal that "activation" of anion transport at high pH by nigericin and at low pH by respiration is associated with an efflux of matrix H+. Other inhibitors of IMAC which are found to block anion uniport in potato mitochondria include propranolol (IC50 = 14 microM, Hill coefficient = 1.28), tributyltin (IC50 = 4 nmol/mg, Hill coefficient = 2.0), and the nucleotide analogs Erythrosin B and Cibacron Blue 3GA. The channel in plant mitochondria differs from IMAC in that it is not inhibited by matrix Mg2+, mercurials, or N,N'-dicyclohexylcarbodiimide. The lack of inhibition by Mg2+ suggests that the physiological regulation of the plant channel may differ from IMAC and that the plant IMAC may have functions such as a role in the malate/oxaloacetate shuttle in addition to its proposed role in volume homeostasis.