Gap junctional communication during the progression of cell cycle from quiescent $G_0$ to S phase was examined in cultured clone 9 rat liver cells. The transfer of scrape-loaded fluorescent dye was suppressed immediately after the stimulation of cell cycle progression in a synchronized cell population. Intracellular pHs in clone 9 cells during $G_0$ / S progression were not virtually altered and thus acidification might be not the cause of gap junctional modulation. Northern blot analysis showed that the temporal disturbance of gap junctional communication in cells passing from $G_0$ to S phase did not result from transcriptional down- regulation of connexin 43. It was also found that the PKC inhibitor, calphostin C, was able to restore intercellular communication in serum stimulated cells. Data suggest a control mechanism by PKC mediated phosphorylation in the regulation of gap junction function which is vulnerable to cell cycling. The loss of gap junctional communication correlated with the increased phosphorylation of connexin 43 on serine residues in clone 9 cells. In an attempt to provide an evidence that phosphorylation of gap junction protein is directly related with the gating of gap junction protein, a single gap junction channel was reconstituted into an artificial liposome. The opening of gap junction channel was examined by means of sedimentation velocities in iso-osmolar density gradient. It was found that connexin phosphorylation by PKC gated the channels and dephosphorylation by CIP restored the gap junction permeability.