We analyzed the direct effects of tyrosine phosphorylation on the structure and functions of tight junctions in temperature-sensitive v-src-transfected MDCK cells. When ts-v-src MDCK cells were plated at a low density at the nonpermissive temperature, they formed small, tight colonies with a typical epithelial appearance. When these colonies were cultured at the permissive temperature, cadherin-based cell adhesion was suppressed, so that individual cells scattered and assumed a fibroblastic appearance. This resulted in the destruction of tight junctions, making it difficult to analyze the direct effects of tyrosine phosphorylation on tight junctions. To suppress cell scattering, we cultured ts-v-src MDCK cells at confluence. Under these conditions, even at the permissive temperature, the cells assumed an epithelial appearance, and the structure of tight junctions were mostly maintained both at the immunofluorescence and electron microscopy levels. The transepithelial electrical resistance (TER) dropped to about 70% of the initial value after the temperature shift from the nonpermissive to the permissive. This temperature shift facilitated the tyrosine phosphorylation of the two tight junction proteins, ZO-1 and ZO-2. We concluded that the direct effects of tyrosine phosphorylation on tight junctions are not so remarkable as those on adherens junctions.