Connexin37 (Cx37) is expressed principally in endothelial cells. We have introduced individual point mutations (Cx37-V156D or Cx37-K162E) in the putative pore lining segment M3 of a polymorphic human Cx37 (Cx37-S319) and expressed them in N2A and RIN cells. RT-PCR and immunofluorescence microscopy were used to confirm the expression of the proteins. Stably transfected cells were subjected to electrophysiological studies. Experiments were performed on cell pairs using the dual whole cell patch-clamp method. Single channel records showed that both mutants display a variety of conductive states (Cx37-V156D, 47-250 pS; Cx37-K162E, 58-342 pS) in contrast to the typical high conductance of 340-375 pS and subconductive state of 60-80 pS reported for Cx37-S319. Analysis of the macroscopic data for Cx37-K162E revealed a broadened Vo indicating the influence of the mutation on voltage gating. Our data indicate that substitution of a conserved residue with a charged residue could cause changes in the main state and/or in the size of the pore. It is possible that these particular residues in the M3 domain interact electrostatistically with several of the other domains in the Cx37 protein.