The effects of mitosis on gap junctional intercellular communication (GJIC) were quantified in a clonal cell line of spontaneously immortalized rat granulosa cells (SIGC) using a fluorescence recovery after photobleaching assay. Reduction of GJIC was associated with the process of mitosis and was first apparent at the onset of prophase. Resumption of GJIC between newly divided cells and surrounding cells occurred slowly, requiring several hours following cytokinesis before reestablishment of maximal rates. Mitotic rates of GJIC in SIGC were comparable to values obtained in interphase cells partially uncoupled by 0.5 mM octanol. Limited studies of other cell lines generalized the mitotic-associated reduction of GJIC observed in SIGC. The data suggest that mitosis is one process which alters GJIC. This could be of significance when there is a change in the rate of proliferation, such as in the acquisition of immortalization, an early stage of transformation.