Electron microscope examinations of the developing triadic junction in fibers from leg muscles of fetal and postnatal rats reveal a range of complexity from no structural connections across the space between apposed membranes of T and SR to all of the junctional structures visible in adult rat muscle fibers. In triads showing no connections, faintly stained projections from the apposed membranes of T and SR are seen. The simplest form of junctional structure is a series of faintly stained strands that make connections across the space between apposing membranes. Junctional structures stained more intensely show periodic bridges traversing the space plus an intermediate dense line (intermediate density) running parallel to the apposed membranes of T and SR and bisecting the space between apposed membranes. The intermediate density is continuous across the periodic bridges and interbridge spaces and it appears to be connected to the apposed membranes of T and SR. It is suggested that projections from T and SR are precursors of the connections and that the projections interact to form the intermediate density. It is also suggested that bridge and interbridge junctional structures might form compartments isolated from the sarcoplasm and thus enhance the flow of electrical current from T to apposed SR by reducing low-resistance shunting of current flow into the sarcoplasm,