Normal rat kidney (NRK) cells cultured in the presence of epidermal growth factor are contact-inhibited at confluent densities. In the additional presence of transforming growth factor (TGF)-beta, however, cells undergo phenotypic transformation which is accompanied by a loss of contact inhibition. In this study, we show by means of the fluorescence photobleaching recovery technique and a scrape-loading dye transfer technique that quiescent confluent cultures of NRK cells do not show extensive gap junction-mediated intercellular communication. Cells contact-inhibited in the presence of epidermal growth factor also show only limited intercellular communication, although with an enhanced permeability coefficient. Cells phenotypically transformed upon addition of TGF beta, however, show extensive intercellular communication, with a similarly enhanced permeability coefficient. This enhanced intercellular communication induced by TGF beta is paralleled by an increase in intracellular pH. It is concluded that in contrast to what has been observed during tumorigenic transformation, phenotypic transformation of NRK cells induced by TGF beta results in an enhancement of the extent of gap junction-mediated intercellular communication.