Connexins are the subunits of gap junction channels which connect neighboring cells. With the exception of lens connexins, they usually do not form open hemichannels in the cell membrane of single cells. Here we describe a chimeric connexin consisting of cx32 where the first extracellular loop sequence is replaced by the corresponding cx43 sequence. This chimera, cx32E143, forms conventional gap junction channels in the paired oocyte assay. In addition cx32E143 induces a membrane conductance in single oocytes. This membrane conductance is voltage dependent and is similarly sensitive to CO2 as are gap junction channels formed by the chimera or by wild-type cx32. These data suggest that cx32E143 forms patent hemichannels in the plasma membrane of single oocytes. This conclusion is further supported by the observation that oocytes expressing cx32E143 take up from the bath medium tracer molecules known to pass through gap junction channels.