Rat ovarian granulosa cells and mouse myocardial cells respond to cell-specific hormones by cyclic AMP-dependent mechanisms. In coculture, these heterologous cells communicate by means of gap junctions. Exposure of the cocultures to a hormone specific for one cell type causes the heterologous cells to respond through a cell contact-dependent mechanism. These studies suggest that this cross-stimulation results from the intercellular communication of a mediator that is common to both cell types. The communicated mediator may be cyclic AMP.