Studies were conducted to determine the effect of culture medium pH on gap junctional intercellular communication (GJIC) in early passage Syrian hamster embryo (SHE) cells. Previous studies have demonstrated that SHE cells cultured at a clonal density at pH 6.70 are morphologically transformed by carcinogens at a significantly higher frequency than cells cultured in media of higher pH. Several other cell characteristics consistent with promotion-like effects are observed with pH 6.70 culture of SHE cells. It was postulated that the promotion-like effects observed in SHE cells cultured at acidic pH are mediated in part by a reduction of GJIC. In this study, we evaluated GJIC in SHE cells by fluorescent dye coupling. Results from this study indicate that GJIC decreased as a function of decreased extracellular pH. Cells cultured at pH 6.70, 7.15 or 7.35 exhibited 47, 75 and 85% coupled cells respectively. The decrease in dye coupling occurred by 24 h after switching the cells from pH 7.15 to 6.70 medium. The decreased GJIC observed at pH 6.70 was not due to changes in cell proliferation and was reversible within 24 h when pH 6.70 cultures were refed with pH 7.15 medium. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate inhibited SHE cell GJIC in a pH-dependent manner with cells at pH 6.70 exhibiting the greatest inhibition by TPA and cells at pH 7.35 being unresponsive. The effect of pH on GJIC in SHE cells is consistent with the pH-dependent response to chemically induced morphological transformation and may be mechanistically related to this phenomenon.