The mitochondrial ADP/ATP carrier, or Ancp, is a member of the mitochondrial carrier family (MCF). It exchanges ADP and ATP between matrix and intermembrane space. It is postulated from numerous experiments that the inactive Ancp bound to one of its inhibitors (CATR or BA) is a dimer, and it is inferred that the active unit is a dimer, too. However, the structure of beef Ancp bound to CATR obtained at high resolution is that of a monomer. To ascertain the dimeric organization of Ancp, we have constructed covalent tandem dimers of which one "subunit" (protomer) is the wild type and the other is inactive for ADP/ATP exchange. We have chosen either the op1 mutant or another member of the MCF, the phosphate carrier (Picp). Activities of the chimeras were first evaluated in vivo. The Ancp/op1 constructs exchange the adenine nucleotides. The Anc/Pic chimeras are considered as bifunctional forms since they exchange ADP and ATP and transport Pi within the same cells. We have then controlled the fact that the chimeras are stable in vivo and in vitro. Proteinase K digestion showed that both protomers of Ancp/op1 have similar organization in the membrane. Analyses of kinetic properties indicated that protomers of Ancp/op1 chimeras crosstalk during the nucleotide exchange unlike those of Anc/Pic. However, full inhibition of phosphate uptake by CATR, a very specific inhibitor of Ancp, strongly suggests that the native functional unit of Ancp, and thus of Picp, is a dimer.