Hepatocyte growth factor/scatter factor (HGF/SF) is a fibroblast-derived protein that affects the growth, motility, and differentiation of epithelial and endothelial cells. We have investigated the effect of HGF/SF on junctional communication in mouse keratinocytes (MK cells). HGF/SF inhibited cell communication in MK cells as assessed by the transfer of a low-molecular-weight dye, Lucifer Yellow. The inhibition was rapid, the earliest effects being apparent 5 to 10 min after addition of the factor, and was transient. The decrease in dye transfer correlated with a loss of the gap junction protein connexin 43 as measured by Western blotting, probably due to increased protein degradation. The results show that junctional communication is an early target of HGF/SF activity and they are consistent with the hypothesis that gap junctions are primary targets of the action of growth factors.