Pod walls of broadbean (Vicia faba L. cv Aguadulce) were harvested at the import (S(1)), at the transition (S(2)) or at the export (S(3)) phase for assimilate transport. Measurements of the transmembrane potential difference (PD) of mesocarp cells were made under various osmotic conditions. Internal osmotic potentials and cell turgor were calculated from osmolality measurements of cell saps recovered by freeze-thawing, after correction for the contribution of the free-space solution. Changes in the mannitol concentration of the medium altered the PD within a few minutes, and new stable values of PD were reached within 20 minutes after the osmotic change. With mannitol as the osmoticum, the most negative PD was measured at an external osmotic potential of -0.70 megapascals (MPa) for S(1) and S(2), while the most negative was at -0.40 MPa for S(3). Ethylene glycol, a permeant osmoticum, had little effect on PD, showing that the PD was sensitive to turgor, not to solute potential per se. For S(1) and S(2), the PD was less negative for turgor potentials lower than 0.1 MPa or greater than 0.3 MPa. S(3) samples exhibited a different turgor dependence, with a sharp optimum of the negativity of the PD at 0.3 MPa. The data are consistent with the proposal that the proton pump acts as a transducer of the osmotic conditions. They show that the osmotic sensitivity of the PD of mesocarp cells of broadbean changes with the stage of development of the pod.