Gap junctions are formed by a family of homologous proteins termed connexins. Their channels are dodecamers, and homomeric forms differ in their properties with respect to control by voltage and other gating stimuli. We report here the properties of coupling from expression of connexin complementary RNAs (cRNAs; sense to mRNA, antisense to cDNA) in Xenopus oocyte pairs in which endogenous coupling was blocked by injection of DNA oligonucleotides antisense to the mRNA of Cx38, the principal endogenous connexin. We found that a connexin recently sequenced from rat liver, Cx26, formed functional gap junctions whose conductance exhibited voltage dependence with unusual characteristics suggestive of two gating mechanisms. Junctional conductance (gj) was increased to a small degree by depolarization and decreased by hyperpolarization of either cell in a coupled pair, indicating dependence on the potential between the inside and outside of the cells (Vi-o). These changes were fast compared with the resolution of their measurement (ca. 10 ms). On a slower timescale, large transjunctional potentials (Vj) of either sign caused a more substantial decrease in conductance similar to that previously reported for several other gap junctions. Homotypic junctions formed of another connexin, Cx32, exhibited a similar slow dependence on Vj but no dependence on Vi-o. In contrast, heterotypic junctions between an oocyte expressing Cx26 and one expressing Cx32 were electrically asymmetric; they exhibited a greater fast change in gj, which depended, however, on Vj, such that gj increased with relative positivity on the Cx26 side and decreased with relative negativity on the Cx26 side. There was also a large slow decrease in gj in response to Vj for relative positivity on the Cx26 side but not for Vj of the opposite sign. These data indicate that properties of the hemichannels contributed by the two connexins in the heterotypic case were changed from their properties in homotypic junctions. The fast change in gj may involve a mechanism analogous to that at fast rectifying electrical synapses. Experiments in which oocytes expressing Cx32 were paired with oocytes expressing both Cx26 and Cx32 demonstrated that asymmetric junctions would form between oocytes expressing both connexins, thereby confirming their potential relevance in vivo, where the same coupled cells are known to express both proteins.